Abstract: To improve the thermal conversion efficiency of the expander applied in the organic Rankine cycle for recovering low-grade waste heat from internal combustion engine,the performance of a free piston expander-linear generator was studied by experiments and simulations in the air test rig. A simulation model built by MATLAB / Simulink software was verified by experiments,and the influence of the initial displacement of the piston,the piston-mover mass and working frequency on the dynamic characteristics was studied. The results show that there is only one point(- 0. 045 307 m) that enables the piston to do reciprocating motion near the displacement of 0 point when inlet pressure and working frequency are 0. 188 MPa and 2. 5 Hz,respectively. Stroke length,peak velocity,peak acceleration and indicated work of the piston decrease with the piston-mover mass increasing. To protect the system of free piston expander-linear generator from damage,the frequency can not be too low,otherwise the piston will hit the cylinder.