A method for controlling a belt drive of a drivetrain of a motor vehicle, with a planetary gear set which shifts a transmission ratio between a starter-generator and a combustion engine, the planetary gear set having a ring gear which can be connected fixedly with respect to a housing by a brake, a web which is connectible to a crankshaft of the combustion engine of the drivetrain, having planet gears arranged around the circumference, and a sun gear which is connected in a rotationally fixed connection to a rotor of the starter-generator and to at least one ancillary unit, as well as having a first switchable free-wheeling mechanism which blocks in the direction of torque from the rotor to the crankshaft and having a second switchable free-wheeling mechanism which blocks in the direction of torque from the crankshaft to the rotor.