1. A fast projection matching method for computed tomography images, comprising the steps of:(a) capturing a first XY projection intensity image of an object on an XY plane with the object rotating at a first angle corresponding to a rotation axis;

(b) capturing a second XY projection intensity image of the object on the XY plane with the object rotating at a second angle corresponding to the rotation axis;

(c) conducting an integral calculation, along an X-axis direction, on the first XY projection intensity image for calculating a first intensity distribution of the object at the first angle;

(d) conducting the integral calculation, along the X-axis direction, on the second XY projection intensity image for calculating a second intensity distribution of the object at the second angle;

(e) comparing whether the first intensity distribution and the second intensity distribution are identical;

(f) when the first intensity distribution and the second intensity distribution are not identical, rotating the first XY projection intensity image along a Z-axis direction and moving along a Y-axis direction to obtain a first XY projection intensity-modified image, and rotating the second XY projection intensity image along the Z-axis direction and moving along the Y-axis direction to obtain a second XY projection intensity-modified image;

(g) conducting the integral calculation, along the X-axis direction, on the first XY projection intensity-modified image for calculating a first intensity-modified distribution of the object at the first angle, and conducting the integral calculation, along the X-axis direction, on the second XY projection intensity-modified image for calculating a second intensity-modified distribution of the object at the second angle;

(h) balancing a difference value between the first intensity-modified distribution and the second intensity-modified distribution;

(i) when the difference value between the first intensity-modified distribution and the second intensity-modified distribution is greater than a first preset value, repeating the Step (f) to the Step (h) until the difference value between the first intensity-modified distribution and the second intensity-modified distribution is smaller than the first preset value, and when the difference value of the first intensity-modified distribution and the second intensity-modified distribution is smaller than the first preset value, defining the first intensity-modified distribution or the second intensity-modified distribution as an initial intensity base distribution;

(j) capturing a third XY projection intensity image of the object on the XY plane with the object rotating corresponding to the rotation axis at every angle from 0 degree to 180 degree by a preset angle interval;

(k) conducting the integral calculation, along the X-axis direction, on the third XY projection intensity image of the object at each said angle for calculating a third intensity distribution of the object at each said angle;

(l) rotating the third XY projection intensity image of the object along the Z-axis direction at each said angle, for producing a plurality of third XY projection intensity-modified images of the object at each said angle;

(m) conducting the integral calculation, along the X-axis direction, on the third XY projection intensity-modified images of the object at each said angle, for calculating a plurality of third intensity-modified distributions of the object at each said angle;

(n) calculating a first minimum difference intensity distribution with the minimum difference from the third intensity distributions and the third intensity-modified distributions to the initial intensity base distribution;

(o) averaging the first minimum difference intensity distribution of the object at each said angle for producing a first average intensity base distribution;

(p) calculating a second minimum difference intensity distribution of the object at each said angle with the minimum difference from the third intensity distributions and the third intensity-modified distributions to the first average intensity base distribution;

(q) averaging the second minimum difference intensity distribution of the object at each said angle for producing a second average intensity base distribution;

(r) determining whether the difference value between the first average intensity base distribution and the second average intensity base distribution is smaller than a second preset value; and

(s) when the difference value between the first average intensity base distribution and the second average intensity base distribution is smaller than the second preset value, rectifying an offset, in the X-axis direction, of the third XY projection intensity image or the third XY projection intensity-modified image in correspondence with the second minimum difference intensity distribution of the object at each said angle, for producing the computed tomography images.