The invention designs an automotive variable transmission ratio steering system based on a hydraulic system. The system has the advantage of accurate control and can obtain hand power feedback in time. The system is composed of a steering wheel (1), a steering shaft (2), a mechanical driving input module (5), a hydraulic loop module (20), an electromagnetic shunt device assembly (13), an oil way pipeline (17), a steering oil pump (19), an oil pressure sensor (22), an electromagnetic flow control valve (21), a steering execution module (15), a wheel angle sensor (16), an electronic control unit (10), a gear shaft (4), a rack (23), a rotary valve (3), a left power cylinder (18), a right power cylinder (6), a left second-level oil cylinder (14), a right second-level oil cylinder (9), an electromagnetic control device (11), a shunt pot (12), a steering transverse pull rod (8) and a piston (7). By means of control of the electronic control unit and implementation of the electromagnetic shunt device, hydraulic oil amount is adjusted to change the steering transmission ratio, and accurate steering control is achieved.