The invention discloses a motor-driven power-assisted steering system with a duly-variable transmission ratio and a control method thereof, belonging to a motor-driven power-assisted steering system. The system comprises a steering wheel (1), a torque sensor (2), a steering motor (3), a planet gear (4), a sun gear (5), a gear ring (6), a steering tubular column (38), a power-assisted motor (7), a retarding mechanism (8), a rack (9), a steering rocker arm (10), a front wheel (11), an electronic control unit ECU (12), an advanced electronic control module ECUa (27), a vehicle-mounted satellite navigator (28), a vehicle-mounted foresight road condition instrument (29) and a self-adaption memory electronic control module ECUb (30). Because a planetary gear mechanism, a steering synchronizing clutch and a steering motor are added between the torque sensor and the gear rack, the motor-driven power-assisted steering system can realize the functions of steering mode selection, duly self-adaption variable transmission ratio control and active steering intervention on the basis of finishing the original motor-driven power-assisted steering; the steering stability and the travelling safety of the automobile can be improved; and the safety and the flexibility of the automobile can be perfectly combined.