Sliding　friction　losses　play　a　significant　role　in　power　losses　of　a　gearbox.　The　author　has　proposed　a　calculation　method　of　sliding　friction　losses　considering　the　transmission　errors.　The　tooth　surface　meshing　force,　relative　sliding/entrainment　velocity,　and　sliding　friction　coefficient　are　obtained　under　static　or　quasi-static　conditions　which　restrict　the　accuracy　of　model　in　the　middle-　and　high-speed　gear　transmission.　The　efficiency　of　dynamic　state　of　a　helical　gear　pair　is　studied,　because　it　is　widely　used　in　the　middle-　and　high-speed　transmission.　Therefore,　a　calculation　method　for　helical　gear　pair　of　sliding　friction　losses　based　on　dynamic　characteristics　is　proposed.　First,　the　dynamic　model　of　helical　gear　pair　considering　friction　and　multiple　backlashes　is　built　and　solved.　The　parameters　associated　with　the　calculation　of　sliding　friction　loss　are　obtained.　Second,　the　tooth　surface　meshing　force　and　relative　sliding/entrainment　velocity　are　calculated　and　brought　into　the　formula　of　sliding　friction　coefficient.　The　formula　of　sliding　friction　losses　is　given,　and　the　meshing　efficiency　considering　the　sliding　friction　losses　in　helical　gear　pair　is　calculated.　Finally,　by　a　comparison　with　Ref.　[1]　(Wang　et　al.,　J　Mech　Eng　Sci,　230(9):1521-1531,　2016),　we　mainly　propose　an　idea　for　calculating　the　sliding　friction　losses　using　dynamics　instead　of　static　method　or　quasi-static　method.