Because　of　the　complicated　structure　of　a　planetary　transmission　system,　the　effect　of　gear　damage　on　the　dynamic　characteristics　is　not　easy　to　assess,　which　has　become　a　main　obstacle　to　the　fault　diagnosis　of　such　a　system.　Based　on　the　precondition　that　the　inner　ring　gear　is　fixed,　this　paper　studied　the　stiffness　synthesis　method　of　healthy　NGW21　planetary　gearbox　and　the　stiffness　degradation　behaviors　induced　by　gear　surface　defect.　The　meshing　stiffness　distribution　curves　of　planet　gear　when　engaging　with　sun　gear　and　ring　gear　separately　were　obtained　by　potential　energy　principle,　then　the　meshing　stiffness　synthesis　method　of　multi-point　meshing　condition　was　established　based　on　the　inner　kinetic　relationship　of　the　gearbox　and　phase　relationship　between　the　planets.　For　the　case　of　sun　gear　and　ring　gear　with　crack　of　different　depths,　the　meshing　stiffness　distribution　of　cracked　gear　meshing　with　planetary　gear　was　discussed,　and　finally　the　stiffness　synthesis　of　the　internal　and　external　meshing　of　the　same　planet　gear　was　completed.　The　analysis　results　show　that　the　crack　of　the　sun　gear　has　more　obvious　effects　on　stiffness　degradation　behaviors　and　will　have　significant　influence　on　the　system　characteristics.　©　2018,　Editorial　Department　of　Journal　of　Beijing　University　of　Technology.　All　right　reserved.