The invention relates to an auxiliary braking system of a motor vehicle, comprising an auxiliary braking wheel device, a power device, a driving device, an elevating device and a control device, wherein the auxiliary braking wheel device comprises a bridge body and auxiliary braking wheels arranged on the bridge body; the input end of the driving device is connected with the power device, and theoutput end is connected with the auxiliary braking wheels in a driving way; the elevating device is arranged on a frame of the motor vehicle and connected with the power device and the auxiliary braking wheel device so as to accept the power of the power device to control the elevation of the auxiliary braking wheel device; when auxiliary brake is needed, according to a control signal sent by thecontrol device, the elevating device is driven by the power device to drive the auxiliary braking wheel device to descend so that the auxiliary braking wheels of the auxiliary braking wheel device contact ground, and meanwhile, the driving device transmits the power of the power device to all the auxiliary braking wheels so that the auxiliary braking wheels rotate opposite to the driving direction of the motor vehicle, thereby generating traction force opposite to the driving direction of the motor vehicle to enhance the brake and stop the motor vehicle.