The operation control strategies of free-piston engine are investigated in this paper. To start the engine, it needs to adjust the compression pressure and the reset valves should cooperate with the frequency control valve to avoid squirm of the piston for hydraulic leakages. The reset valves should be closed before frequency control valve is opened. A piston position feedback modulation control method which can make sure the piston accelerated continuously is presented. It has fixed frequency control valves open signal moment but no fixed frequency control valves closed signal moment, and the closed signal was activated by piston position feedback.