The　force　analysis　of　parallel　mechanism　or　complex　mechanism　by　using　the　traditional　method,　especially,　when　it　needs　to　calculate　constraint　reactions　of　all　kinematic　pairs,　is　quite　inconvenient　because　a　large　number　of　simultaneous　equations　need　to　be　solved.　Using　screw　theory　to　analyze　such　mechanisms　will　simplify　the　problem.　With　this　method,　it　can　develop　the　force　analysis　of　the　mechanism　including　active　forces　under　external　force　and　constraint　reactions　of　all　kinematic　pairs,　and　it　can　deal　with　the　active　forces　and　constraint　reaction　forces　of　every　kinematic　pair　respectively.　The　advantage　is　that　the　method　can　ensure　the　number　of　equilibrium　equations　being　no　more　than　six　each　time.　For　understanding　the　characteristics　of　this　force　analysis　method　based　on　screw　theory,　a　single-loop　spatial　mechanism　is　taken　as　an　example,　which　consists　of　two　revolute　pairs　(R),　a　spherical　pair　(S)　and　a　universal　pair　(U),　i.e.　RSUR　mechanism.　It　needs　to　solve　its　active　forces　and　constraint　reactions　of　all　kinematics　pairs　with　this　method.　Through　the　solution　it　can　be　seen　that　the　method　greatly　simplifies　the　process　of　calculating　the　force　analysis　and　the　solution　process　is　clear　and　easy　to　understand.　©2011　Journal　of　Mechanical　Engineering.