The invention discloses a commercial vehicle electrical auxiliary braking system and method. The auxiliary braking system comprises a motor, a controller of the motor, a braking resistor and a storage device; electric energy generated by the motor in the feedback braking process is used for charging the storage device through the controller or consumed through the braking resistor. According to the commercial vehicle electrical auxiliary braking system, when a vehicle goes up and down on a slope, different auxiliary braking schemes are selected according to different vehicle speeds, the braking efficiency stability is effectively improved, and the operating stability and driving safety of the vehicle are guaranteed. Accordingly, layout of a braking resistor cooling device is optimized, a bypass is additionally arranged on a motor cooling system, the degree of freedom of the system is improved, under the low-speed downhill working condition, when the motor is not needed for braking, the motor stops, braking is assisted only through the motor, and the energy-saving purpose is achieved to a certain extent.