1. A microwave cooking device, comprising:a shell defining an air inlet and an air outlet therein;
a filter screen disposed at the air outlet; and
a ventilation grid assembly comprising:
a body defining a through hole therein and disposed on an outer surface of the shell, the through hole being opposite to the filter screen;
a cover plate disposed on the body and moveable between an open position where the through hole is opened by the cover plate and a closed position where the through hole is closed by the cover plate, wherein the cover plate defines a positioning hole and a snapping groove therein;
a first support disposed at a bottom wall of the through hole;
a positioning column defining a first end disposed on the first support and a second end which is a free end and is fitted within the positioning hole, the cover plate being rotatable relative to the second end of the positioning column;
a second support and a third support disposed at the bottom wall of the through hole and spaced apart from each other; and
a positioning pin defining a first end disposed on the second support and a second end disposed on the third support, and snap-fitted within the snapping groove, the cover plate being rotatable relative to the positioning pin.