Abstract: To increase the system efficiency and reliability, performances of distributing valves on a hydraulic free-piston engine were studied. Dynamic models of the valves combined with a full-cycle engine model were built and a test apparatus was set up. The relations between dynamic responses of the valve spools and the pressure changing in pump chamber were analyzed via simulation and test. The influence of distributing valves' working characteristics on the system performance was investigated. The results indicate that faster close response of the inlet valve increases the engine fuel combustion efficiency. The response lag between the outlet valve and the piston around bottom dead centre decreases the system controllability in low working frequency. A high velocity impact between valve spool and caging device might appear in the valve opening process, which makes a fall in valve operating life.