The invention provides a shift actuator of a transmission, wherein the switching of two gears and one neutral position of the transmission is realized by utilizing three positions where a two-stroke cylinder piston can stay. Meanwhile, the current gear of the transmission and the fault messages of an actuator can be accurately judged by utilizing a position sensor arranged on a cylinder. A shifting actuator used for gear shifting of the transmission comprises the cylinder and pistons which are arranged inside the cylinder and can move back and forth, wherein a piston rod is movably connected to a transmission gearshift. The shifting actuator is characterized in that the pistons are divided into a front piston and a back piston, the front piston and the back piston move in a fixed stroke, and the cylinder is divided into three spaces by the front piston and the back piston; a first end port is formed in the first space for gas transmission to the first space; a second end port is formed in the second space for gas transmission to the second space; a third end port is formed in the third space for gas transmission to the third space.