Abstract: The dynamics of the hydraulic valves in a hydraulic free-piston engine is given. The piston moving characteristic a single piston hydraulic free piston engine was given and the hydraulic valves formats were presented. The mathematical model of the hydraulic valves was established. The stabilities, frequency responses, spool displacement time response and flow characteristics of the hydraulic valves were analyzed. The results indicate that the suck and vent valves should be backward and forward outward flow cone valves, respectively. Reduction of the spool moving mass and viscous damping coefficient minifies the phase offset between the spool and the piston movements and leads to the increase of system volumetric efficiency. The instantaneous opening of the vent valve in the opening process is large and the spool movement is affected by the stop device.