The present invention sucks the air from the fan at the top of the imaging assembly of the optical engine and blows the air via the air duct device downward. Some of the air is guided by the diversion board, aslant guiding surface, and aslant isolating board of the air duct device and blown into the imaging assembly for cooling the optical components. Some of the air is guided through a first air duct, a second air duct, and a third air duct extended from the outside of this main body respectively to the outer surface of the light valves. Further, some of the airflow is guided into a branch air duct and blown to a vent of the imaging assembly. By means of the changing direction diversion board, the airflow is guided towards the polarizer module, and flows out from a vent on the other side.