A free-piston linear alternator with an axial uniflow-scavenged using passive charge valve was designed. The structure and principle of the alternator were introduced. A simulation model of the working process which was validated with the test was established based on dynamics and thermodynamics equation. The motion characteristics and the performance effective factors of the alternator which was beneficial for performance optimization and prototype design were analyzed. The simulation results show that the alternator possesses a longer ventilation time and the duration accounts for about 58% of the whole cycle, which provides sufficient fresh mixture entering the cylinder. The motion curve of piston is non-symmetrical, the acceleration of the free-piston at TDC changes violently and it needs accelerate the combustion heat release rate to adapt to the changes. It should appropriately increase the piston mass and the opening-closing position of the exhaust valve, and reasonably reduce the ignition advanced position and the working pressure of the inlet valve to reach higher output power.