The invention discloses a car engine BSG start and stop control method and device and relates to the technical field of cars. According to the method, when a crankshaft is combined with a pulley system, the pulley system is driven by the crankshaft, and the BSG is in a power generator state; when the crankshaft is separated with the pulley system, the BSG is in a motor state and operates in the preset rotating speed, and the normal operation of attachments are maintained. An additional sensor is omitted, the original pulley system is free of changing, the ideal fuel saving effect is achieved, the entire car cost is low, the development period is short, and the production investment is low. The device comprises a BSG pulley, and the BSG pulley, a power assisted steering pump pulley, a water pump pulley, an air conditioning compressor pulley and a crankshaft pulley are connected through a belt; the entire original engine pulley system and transmission are maintained, the pulley system is free of changing, the development of the pulley system is omitted, and only an one-way bearing is arranged in the hub portion of the original crankshaft pulley as required.