To test and verify the reliability of relay valve （with output pressure of 12.0 MPa and response time of 0.2 s）, and to study the influence of dynamic characteristics of the relay valve on braking performance of the full hydraulic braking system, taking a type off-road vehicles fully hydraulic braking system as the research object, a theoretical analysis model of the relay valve was established. A simulation model of the full hydraulic brake system was established by applying the software AMESim, and the influence of the spool friction, initial cover, return spring the initial amount of compression and spring stiffness on braking performance was analyzed. The accuracy of the simulation model was verified by experimental results. The comparison shows that the relay valve applied to the hydraulic braking system can meet the braking requirements（output pressure of 12.0 MPa and response time of 0.2s）. Excessive friction of the spool increases the opening pressure of the relay valve, which leads to the increase of proportional hysteresis of the relay valve and affects the reset performance of the spool. The greater the initial cover of the relay valve orifice is, the greater the friction to be overcome by opening the orifice becomes, and the longer the response time of the braking system is required. By adjusting the initial compression of the reset spring of the relay valve and the stiffness of the spring, the fine tuning of the brake pressure can be realized. The theoretical model and simulation model provide a reliable basis for further optimization of the full hydraulic braking system.