Abstract: Abstract: Free-piston engine generator (FPEG) contains two gasoline engines and a linear generator, which has attracted considerable research interest for its potential application to electricity generation. The device has low mechanical loss and variable compression ratio which means energy conservation and multiple fuel adaptability. Previous studies have found that the scavenging process was vital to the stable operation of the FPEG system. In order to know the features, an experiment was carried out to analysis the scavenging process of FPEG. In the experiment, both the pressure in cylinder and in scavenge box were tested. Experiment results showed that: the combustion process of FPEG variated cycle to cycle at low speed operation condition for the instability of scavenging process, while in traditional mode the engine performed very well. Computational fluid dynamics (CFD) model designed according to prototype was introduced to analysis the difference between FPEG and traditional two-stroke engine in scavenging process. Calculation results showed that: the relative low scavenging box volume variation rate of FPEG was the effective factor on the scavenging efficiency, which led to low-speed instability. And the difference of scavenging structure between FPEG and traditional engine was responsible for it. In order to optimize the scavenging system, operation parameters including diameter of piston assembly, volume of scavenging box and charging pressure were investigated. Results showed that: the low proportion of piston assembly diameter on bore and high inlet pressure effectively improved the scavenging efficiency. Especially, when the proportion of piston assembly diameter on bore was less than 0.4, the difference of scavenging efficiency of FPEG and traditional engine was less than 6%.