For　proactively　arranged　downtime　and　maintenance　of　the　gearbox,　the　accurate　assessment　of　residual　useful　life　(RUL)　for　worn-surface　gear　is　a　critical　problem　that　needs　to　be　solved.　In　light　of　this,　a　novel　dynamic　assessment　methodology　for　RUL　of　sun　gear　was　proposed　by　coupling　Archard＇s　wear　model　and　a　nonlinear　dynamics　model.　By　modifying　the　internal　excitation　model,　the　effect　of　tooth　surface　wear　is　included　in　the　dynamics　model.　Then,　the　effect　of　dynamic　behavior　on　the　wear　process　of　the　tooth　surface　is　further　fed　back　by　the　dynamic　meshing　force.　On　this　basis,　the　influence　of　input　speed,　torque　load　and　initial　wear　depth　on　the　RUL　was　analyzed　using　a　planetary　gear　NGW11-6　as　an　example.　The　results　indicate　that　the　torque　load　has　the　greatest　influence　on　the　RUL,　followed　by　the　initial　wear　depth,　and　that　the　input　speed　has　the　least　influence.　The　established　methodology　will　provide　a　theoretical　basis　for　the　RUL　assessment　and　scheduled　maintenance　of　transmission　systems　with　worn-surface　gears.