To further understand single-piston free piston expander linear generator (FPE-LG), a novel FPE-LG prototype had been established for small scale organic Rankine cycle(ORC) waste heat recovery system. The single-piston FPE-LG can operate continuously and stably based on the compressed air test bench, which indicates that the working principle of single-piston FPE-LG is feasible. The effects of intake pressure, piston stroke length, external load resistance and intake duration on the motion characteristics of the single-piston FPE-LG are investigated, and the results show that the variation trend of piston displacement and velocity exhibits a regular sinusoidal fluctuation. Relative high velocity is near the dead centers and peak acceleration is usually obtained when the piston arrives at left dead center(LDC) or right dead center(RDC). By using the variable length of piston stroke to improve output power is feasible. But the change of piston stroke has little effect on the peak piston velocity and the peak pressure of cylinder. The peak piston velocity and piston stroke utilization ratio (η) increase with the external load resistance. When the external load resistance increases from 20 Ω to 80 Ω, the peak piston velocity and η increase from 0.56 m/s and 58.26% to 0.84 m/s and 78.42%, respectively. Increasing intake duration is beneficial to obtaining higher piston velocity, piston stroke utilization ratio and output power. The peak output power increases from 13.87 W to 37.40 W when the intake duration changes from 20 ms to 40 ms.