The invention discloses a constant-speed unloading mechanism. The constant-speed unloading mechanism comprises an unloading valve body and an unloading valve cover, wherein the unloading valve cover is arranged at the upper end of the unloading valve body, a piston and an unloading valve plate are arranged in the unloading valve body, the unloading valve plate is arranged at the lower end of the unloading valve body, an unloading claw is further arranged at the lower end of the unloading valve plate, and a reset large spring is further arranged between the unloading valve plate and the piston;and an air inlet and an air chamber are formed in the unloading valve cover, and the piston is connected with the air chamber. According to the constant-speed unloading mechanism, the purpose of non-stop unloading of a compressor is achieved, and the problems of overload relay tripping, motor burnout and the like possibly occurring due to frequent starting of the compressor are solved. When the compressor is not shut down, the piston is impacted by gas in the unloading valve cover, the unloading claw of the unloading valve plate presses the air inlet valve plate of the head part of an air valve air cylinder slowly, the air inlet valve is in an open state, and the compressor is in a constant-speed unloading state.