In order to accurately control the frequency of single piston hydraulic free-piston engine, it is important to confirm the moving mechanism of hydraulic free-piston around bottom dead centre. Through the establishment of mathematical simulation model and experimental system, the piston motion around bottom dead centre and the influencing factors were investigated. Results indicated that the piston motion around bottom dead centre included a backward acceleration process and a forward deceleration process. The rebound distance was determined by these processes, and the backward accelerative force was applied by the pressures of pump and compression chamber. The controllable pressure for compression chamber is the foundation for accurate operating frequency control, and the pressure is affected by the piston motion and the motion characteristics of the check valve spool.