The invention discloses a vehicle braking system and an auxiliary braking device thereof. The auxiliary braking device comprises a microcontroller, a braking state detecting element configured to detect a vehicle braking state and send the detecting information to the microcontroller, an execution unit controlled by the microcontroller, a motor, a speed reduction mechanism for reducing the speed of output rotation of the motor, an execution element driven by the speed reduction mechanism, and a driven piece arranged in the middle of a vehicle braking wire and joined with the execution element, wherein the motor can drive the execution element to move by the speed reduction mechanism, so that the execution element acts on the driven piece, and the anti-locking acting force opposite to the braking force of the braking wire is generated in the braking wire or the auxiliary braking force is generated in the braking wire in dependence on the movement direction of the execution element. The device can reliably realize the wheel locking prevention in vehicle braking, and can realize the linkage braking.