To　enlarge　the　fault　tolerant　workspace　when　robot＇s　fault　joint　is　locked,　and　improve　the　fault　tolerant　performance　index,　this　paper　used　the　influence　factor　as　the　fault　tolerant　index　to　conduct　structure　synthesis.　Then,　based　on　this　index　an　optimization　method　of　structural　parameters　is　proposed,　which　can　determine　the　link　length　with　optimal　fault　tolerance　for　a　robot.　Simulation　examples　for　a　spatial　4R　robots　are　given.