An automatic compression mechanism driven by a stepper motor comprises the stepper motor, a guide rail, a coupling, a transmission screw rod, a first rolling bearing, a compression plate, a connectingrod, a double-bevel jacking block, a retaining block, an upper jacking plate, a lower jacking plate, an upper sliding block, a lower sliding block, a short guide rail, a short guide rail sliding block and a first reset spring. The stepper motor is connected with the transmission screw rod through the coupling; the upper portion of the transmission screw rod is connected with the first rolling bearing; a transmission nut is connected with the double-bevel jacking block connected with the upper jacking plate and the upper sliding block; the upper jacking plate is connected with the connecting rod, and the connecting rod is connected with the compression plate and the lower jacking plate; and the retaining block is connected with the short guide rail sliding block. By controlling the rotating speed of the stepper motor, the garbage compression time is reduced through a variable speed compression method with the speed being high firstly and then being low, and convenience is brought to auser.