The temperature change processes of functioning parts of the united braking system consist of eddy-current retarder and primary brakes were analyzed to put forward the optimal function for the braking force distribution of the united braking system, and the value of the braking force distribution parameter was suggested. The braking process simulation of an actual passenger car on a virtual multi-gradient ramp was carried out, and the results showed that the final temperature of eddy-current retarder rotor and disc brakes and drum brakes of the united braking force with optimal distribution parameter is relatively low, and varies in a uniform trend. The optimal result of braking force distribution parameter can be refered to for the engineering practice requirements.