Abstract: As the piston movement is of great importance to the in-cylinder flow process, the piston movement characteristics of the diesel free-piston linear generator (FPLG) were analyzed with the equivalent speed transform method. Based on the analysis results, the influence of the piston movement on the in-cylinder flow was studied by using theoretical calculation and three-dimensional numerical simulation respectively. The results demonstrate that compared with the traditional engine, the equivalent speed of FPLA is higher both near TDC and BDC, but lower at the midterm of the stroke. It leads to a higher peak value of squish flow, shorter duration and different curves of mean turbulence kinetic energy.