The　heat　pump　system　with　economizer　coupled　with　scroll　compressor　is　presented.　The　compression　work　of　the　process　with　gas　injection　is　analyzed　based　on　the　first　thermodynamic　law　to　the　system　with　variable　mass,　and　the　relationship　between　compression　work　and　the　mass　and　temperature　of　injecting　refrigerant　is　derived.　The　shortage　of　compression　ratio　of　single　stage　compressor　can　be　compensated　through　refrigerant　injection.　The　calculated　results　show　that　the　heat　pump　system　can　enlarge　the　applying　area　of　air-source　heat　pump.