Two　fault　tolerant　performance　indexes,　reduced　condition　number　and　influence　factor　of　fault　tolerant　workspace　are　introduced.　The　former　indicates　the　manipulability　of　a　reduced　robot　at　the　moment　of　a　failure.　The　latter　indicates　the　relative　workspace　of　the　reduced　robot　at　this　moment.　These　indexes　can　quantitatively　evaluate　the　fault　tolerant　ability　of　a　robot　at　the　moment　of　a　failure　and　after　the　failure　respectively.　Then　based　on　these　indexes　an　optimization　method　of　structural　parameters　is　proposed,　which　can　determine　the　link　length　with　optimal　fault　tolerance　for　a　robot.　Finally,　simulation　examples　are　demonstrated　and　the　sensitivity　of　fault　tolerance　of　a　robot　to　each　link　is　analyzed　with　a　planar　3R　and　a　spatial　4R　robot　respectively.　The　results　show　that　the　fault　tolerant　ability　of　a　robot　is　more　sensitive　to　the　link　that　is　not　directly　connected　with　any　failed　joint　or　is　connected　with　only　one　failed　joint,　which　means　that　these　links　should　be　considered　in　the　first　place　when　adjusting　link　length.　In　addition,　for　spatial　robots　the　corresponding　relation　between　the　two　indexes　is　not　as　regular　as　planar　robots,　and　the　conflict　between　them　is　not　obvious.　This　implies　that　it　is　more　possible　for　spatial　robots　to　simultaneously　optimize　these　two　indexes.