Abstract: In view of requirement of over-speed protection or rope breaked protection of underground track haulage, the paper put forward a solve scheme of automatic hydraulic braking system of vehicle. It introduced principle, functions of components of the system. The system uses high-precision hydraulic valve to test, extract, filter, amplify, choice speed signal of tram, then to complete clamping rail brake through brake valve. The practical application showed that the system can on-line measure speed of tram and has advantages of stable and reliable working, high measurement accuracy, easy operation, long working life.