Charging characteristics of accumulator play an important role in the safety and reliability of hydraulic braking system. The charging characteristics of an electro hydraulic braking system and its key structural element, priority unloading valve, were studied. In the charging process of accumulator of the braking system, the mechanisms of the priority unloading valve and its system components were analyzed to establish the mathematical model for it and to build the simulation and test platform for the charging system. The dynamic characteristics of the accumulator charging system were studied; the changing laws of the parameters such as pressure, flow and time, were gained; and the response law of the priority valve to accumulator charging characteristics was revealed. The research results show that the designed priority unloading valve can meet the requirements of accumulator charging characteristics.