In　order　to　solve　the　problem　of　the　unequal　between　internal　and　external　pressure　ratio　in　single　screw　refrigeration　compressor　under　different　environmental　conditions　and　improve　the　efficiency,　compound　slide　valve　capacity　control　mechanism　was　proposed　in　this　paper.　The　compound　slide　valve　capacity　control　mechanism　can　achieve　capacity　and　internal　volume　ratio　regulation　simultaneously.　On　the　basis　of　the　operating　feature　of　single　screw　compressor,　the　structural　parameters　of　a　single　screw　refrigeration　compressor　were　determined,　and　then　the　influences　of　the　starting　position　of　the　by-pass　and　string　gas　groove　on　internal　volume　ratio　under　different　load　conditions　were　analyzed.　A　model　was　established　to　calculate　the　effective　by-pass　area,　which　laid　the　foundation　for　the　study　of　the　part-load　working　characteristics,　and　the　influences　of　different　part-load　conditions　and　slide　valve　starting　position　on　effective　by-pass　area　under　different　load　conditions　were　obtained.　Results　show　that　the　compound　slide　valve　can　achieve　internal　volume　ratio　regulation　under　full-load　condition,　and　internal　volume　ratio　increases　with　the　decrease　of　capacity　under　part-load　conditions.　String　gas　groove　can　ease　its　increase　effectively.　Jumping　value　increases　with　the　closer　of　slide　valve　starting　position　to　exhaust　end.　Effective　by-pass　area　and　duration　increase　with　the　decrease　of　load.　In　order　to　increase　the　performance　of　part-load　conditions,　the　starting　position　of　by-pass　should　be　as　close　as　possible　to　inlet　end,　but　the　starting　position　of　by-pass　can＇t　cross　the　closed　helix.　©　2018,　Editorial　Department　of　Journal　of　Beijing　University　of　Technology.　All　right　reserved.