The invention discloses a control mechanism for a variable transmission ratio steering system. The control mechanism comprises a control switch, a control switch rack, a slidable rotating shaft, a variable length lever, a steering wheel rack, a planet gear frame, a gear ring, a sun gear, planet gears, a control cable, a motor and a passing gear. The sun gear and the steering wheel rack are engaged with each other. One end of the variable length lever is hinged to the steering wheel rack, and the other end of the variable length lever is connected with the control switch. The slidable rotating shaft can only slide up and down in a groove. The passing gear is engaged with an external gear of the gear ring and the control switch rack. The motor is engaged with the external gear of the gear ring. The planet gears are engaged with an internal gear of the gear ring and the sun gear. According to the control mechanism for the variable transmission ratio steering system, variable transmission ratio transmission of a steering mechanism of an automobile can be achieved, the transmission ratio of the steering mechanism is decreased during a low speed, the sensitivity is improved, the transmission ratio of the steering mechanism is increased during a high speed, the sensitivity is reduced, and the stability during driving is improved; and meanwhile, an angle sensor is not needed, so that the advantage of low cost is achieved.