In　view　of　the　time-varying　pressure　angle　characteristics　of　S-gears,　a　dynamic　model　of　planetary　S-gear　train　which　is　closer　to　the　actual　working　condition　is　established　in　this　paper.　Firstly,　the　machining　method　of　internal　and　external　S-gears　is　introduced　and　the　equation　of　tooth　surface　is　calculated.　Subsequently,　the　pressure　angle　variation　law　of　S-gears　is　derived　according　to　the　line-of-action　(LOA)　equation,　then　introducing　it　into　the　planetary　S-gear　train　to　establish　a　dynamic　model　considering　time-varying　pressure　angle.　Finally,　the　dynamic　response　of　the　planetary　S-gear　train　is　obtained　by　using　the　Runge-Kutta　method,　and　the　influence　of　time-varying　pressure　angle　excitation　on　the　dynamic　characteristics　of　the　system　is　revealed　by　comparing　the　dynamic　mesh　force　(DMF),　gear　mesh　deformation,　and　vibration　displacement　of　components.　The　results　clearly　indicate　that　the　time-varying　pressure　angle　has　a　significant　impact　on　the　dynamic　response　of　the　planetary　gear　train,　which　cannot　be　ignored.　The　research　results　provide　a　useful　source　of　reference　for　the　dynamic　design　and　vibration　control　of　non-involute　planetary　gear　trains.