A　detailed　insight　into　the　liquid　film　characteristics　in　the　helical　channel　of　single　screw　expander　is　of　great　significance　for　avoiding　the　leakage　loss　of　higher　pressure　gas　to　lower　pressure　region.　The　liquid　film　thickness　and　fluctuation　characteristics　of　the　annular　flow　on　the　outer　side　of　the　helical　rectangular　channel　is　measured　using　the　digital　image　processing　algorithm,　and　they　are　evaluated　under　various　flow　rates,　dimensionless　curvature,　and　dimensionless　pitch　at　different　angle　positions.　Results　show　that　increasing　the　Reg　will　decrease　the　instantaneous　liquid　film　thickness,　and　a　larger　decrease　in　the　liquid　film　fluctuation　is　found.　A　larger　average　liquid　film　thickness　is　located　at　the　angle　of　90　degrees　than　that　of　the　180　degrees　and　270　degrees.　The　instantaneous　and　average　liquid　film　thicknesses　are　decreased　with　increasing　the　dimensionless　curvature　and　dimensionless　pitch　at　each　angle　position.　Moreover,　increasing　the　dimensionless　curvature　can　weaken　the　effect　of　angle　position　on　the　liquid　film　thickness.　The　time　distribution　of　the　liquid　film　thickness　changes　dramatically　in　both　wave　height　and　amplitude　as　increasing　the　Rel　at　the　smaller　dimensionless　pitch.