Based on the operating characteristics of a micro HCCI (Homogeneous Charge Compress Ignition) free-piston engine, a multidimensional model which coupled CFD code, chemical dynamic model and piston dynamic model has been established. Using this model, an ignition and combustion process of the micro engine is simulated, the cylinder pressure and temperature profiles are obtained, and the influence of leakage and heat lost on micro engine combustion process is analyzed. Meanwhile, working characteristics of micro engine generator are evaluated by employing the simulation result. Power, combustion efficiency, etc of the micro engine are obtained. Lastly, the micro engine working characteristics with different load, fuel and piston mass are compared and effects of those varying conditions on engine performance are investigated. Simulation result provides essential requirements for micro free-piston engine control and design.