The invention discloses an air exhaust mechanism for a granary ventilation system. The air exhaust mechanism comprises a fan housing and a main housing opening and closing door, wherein the fan housing is arranged in a ventilation duct of a granary wall body and looks like a circular truncated cone; a housing air duct is arranged in the fan housing; the small-diameter end of the fan housing is a fan housing air inlet end communicated with the inside of a granary; the large-diameter end of the fan housing is a fan housing air exhaust end communicated to the outside of the granary; a motor and an impeller are arranged in the housing air duct; the impeller is coaxially connected with a rotating shaft of the motor; a housing sealing ring is arranged between the fan housing and the ventilation duct of the granary wall body; the main housing opening and closing door is used for closing the fan housing air exhaust end; one side of the main housing opening and closing door is hinged to the granary wall body. The air exhaust mechanism has the beneficial effects that the structure is reasonable; air can be effectively guided to realize granary ventilation, and shock absorption and noise reduction can be effectively realized; the structure is firm, and the working process is high in stability; the heat dissipation capacity is high, switching between external circulation and internal circulation can be performed, and thus the air exhaust mechanism is applicable to various heat dissipation and fumigating insect killing modes.