Abstract: 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.