The rack tests of automobile braking effect only give the static parameters. It cant insure the part and reason of default. According to the working principle and the feature of structure of hydraulic braking system, the paper establishes the diagnosing matrix, standard etc. The paper also develops the software of data acquiring, data processing, malfunction diagnosing, and displaying. This system can be used to diagnose the braking device of Xiali TJ7100, and the system can improve the efficiency and the accuracy of trouble diagnosis.