An automatic variable-speed motor transmission mechanism for an electric tricycle and an electric vehicle comprises a motor, an automatic speed changer and a differential system. The automatic variable-speed motor transmission mechanism is characterized in that the automatic speed changer consists of a speed changer shaft, a driving gear, a sun gear fixing plate, a driving disc, a housing and a connecting plate, a motor shaft of the motor drives the speed changer shaft, the connecting plate is mounted at the other end of the speed changer shaft and connected with the housing, a planetary gear is mounted on the upper portion of the housing, the driving disc with a shoe is fixedly connected to the lower portion of the planetary gear, a tension spring is connected with the shoe, the driving disc is sleeved on the outside of the speed changer shaft by a driving disc spline, the planetary gear matches with a sun gear, the sun gear matches with the sun gear fixing plate, through holes on two sides of the sun gear fixing plate are fixed on a body of a gearbox, the driving disc is connected with the driving gear by a driving gear spline, the driving gear drives an intermediate gear, an intermediate driven gear drives a driven gear, the driven gear is connected with the differential system, and a differential spline sleeve is connected to a rear axle shaft of the electric vehicle.