To　reduce　the　sudden-change　of　joint　relative　velocity　by　using　the　reduced　condition　number　as　the　fault　tolerant　index　when　the　robot＇s　fault　joint　is　locked.　Simulation　of　a　planar　3R　and　a　spatial　4R　robots　are　conducted　to　analyse　and　research　the　main　factors　that　influence　the　Relative　Joint　Sudden-Change.　Research　results　show　that　the　fault　tolerant　indexes,　the　location　of　the　end-effector,　the　velocity　of　the　end-effector,　the　initial　joint　angle　and　the　end-effector　trajectory　influence　the　Relative　Joint　Sudden-Change.　Finally,　methods　that　reduce　the　sudden-change　of　joint　relative　velocity　are　proposed.