In order to overcome the serious mechanical vibration in the existed single-cylinder four-stroke free-piston generator, a novel opposed-piston four-stroke free-piston generator is designed and simulated in this paper. The system structure and working principle of the system is discussed. Energy balance equations of free-piston reciprocating motion are derived. Energy flow characteristics of the four strokes are analyzed. A virtual prototype is designed, and the dynamic characteristics and performance of the system is researched by mathematical simulation. According to the research, the proposed opposed-piston four-stroke free-piston generator has advantages of low mechanical vibration and high power density. It is a better choice for hybrid electrical vehicles than the existed design using as power generating systems.