Adopting　an　economized　vapor　injection　compressor　can　improve　the　heating　capacity　and　energy　efficiency　ratio　of　the　heat　pump　system　at　low　environment　temperature.　In　this　paper,　a　kind　of　new　vapor　injection　port　structure　located　at　the　surface　of　slide　valve　was　proposed　based　on　the　single　screw　compressor,　so　the　vapor　injection　technology　also　can　be　used　under　part　load　condition　and　we　proposed　a　new　kind　of　single　slide　valve　structure　called　compound　slide　valve　to　adjust　the　capacity　and　internal　volume　ratio　of　compressor　simultaneously.　A　new　mathematical　method　was　presented　to　study　the　vapor　injection　process.　The　actual　capacity　and　internal　volume　ratio　at　different　slide　valve　working　locations　can　be　determined.　The　pressure,　temperature　and　mass　of　the　working　medium　in　the　control　volume　are　calculated　in　this　study.　The　mathematical　expression　of　the　injection　region　envelope　is　also　presented　in　this　paper.　©　2019　International　Institute　of　Refrigeration.　All　rights　reserved.