The invention discloses an automatic transmission. The automatic transmission is characterized in that a main input shaft, a clutch, a motor and an inner hub input shaft are mounted in a housing of the automatic transmission; the clutch comprises an inner hub, an outer hub and a friction plate assembly mounted between the inner hub and the outer hub, the outer hub is fixedly connected with a rotor input shaft, and the inner hub is fixedly connected with the inner hub input shaft; the outer circumferential part of a piston of the clutch is a flat-plate-shaped working part, the working part corresponds to the working surface of the friction plate assembly in position, a shaft sleeve part of the rotor input shaft is sleeved with the inner circumferential part of the piston, a barrel-shaped part is arranged between the outer circumferential part and the inner circumferential part and surrounds the shaft sleeve part to form a cavity, and inner circumferential part of the piston axially extends into space defined by the inner hub. The space in the radial direction of the clutch is used sufficiently, and smaller axial size of the automatic transmission is realized; the automatic transmission has good universality and is more applicable to a hybrid electric vehicle.