In　this　paper,　a　free　piston　expander-linear　generator　(FPE-LG)　test　rig　is　established　and　the　control　strategy　based　on　displacement　is　applied.　The　operation　characteristic,　output　performance,　and　energy　conversion　efficiency　of　the　FPE-LG　are　analyzed.　The　coefficient　of　equilibrium　oscillation　(C)　is　identified　to　evaluate　the　stability　of　the　free　piston　assembly　(FPA).　Experimental　results　show　that　the　FPA　has　best　stability　when　fixed　position　point　(FPP)　is　between　20　-　75　mm.　The　actual　stroke　and　operation　frequency　increase　with　the　intake　pressure　and　load　resistance.　The　variations　of　voltage　and　power　output　are　in　accordance　with　velocity.　The　average　power　output　increases　with　intake　pressure　and　FPP,　but　reduces　with　the　advanced　exhaust　duration.　The　maximum　average　power　output　can　be　achieved　when　the　load　resistance　is　20　Omega.　The　energy　conversion　efficiency　first　increases　with　intake　pressure　and　then　remains　constant.　The　energy　conversion　efficiency　reaches　up　to　17.5　%　when　intake　pressure　is　0.9　MPa,　FPP　is　40　-　55　mm,　advanced　exhaust　duration　is　0　ms,　and　load　resistance　is　20　Omega.　The　lower　the　coefficient　of　equilibrium　oscillation　is,　the　higher　the　average　power　output　is.