The invention provides a ventilation device capable of automatically adjusting ventilation quantity. In the ventilation device, air vents are respectively formed in two tightly clung ventilation plates; when the air vents in the two ventilation plates are locally or wholly overlapped, the ventilation is realized and the ventilation quantity can be adjusted according to the requirements; and when the air vent in one ventilation plate is covered by the other ventilation plate, the ventilation is stopped.