A pulley assembly for a high-speed continuously variable transmission includes a drive pulley, a shaft sleeve, a movable pulley, rollers, and a back pressure disc. The movable pulley has a shaft portion fitted on the shaft sleeve. The movable pulley has a disc chamber radially formed with multiple pairs of retaining wall pieces. A receiving room is defined between each pair of the retaining wall pieces. The bottom of the receiving room has an inclined push surface. The rollers are received in the receiving rooms. The back pressure disc includes a disc body having a complete inner conical surface. The back pressure disc is subject to the shaft sleeve and unable to be axially displaced along an output shaft. The peripheral edge of the disc body is embedded into the disc chamber of the movable pulley. The inner conical surface is pressed against the rollers.