Robot　joint　is　the　main　actuator　for　the　attitude　control　of　robot　system,　which　has　an　important　effect　on　the　performance　of　the　whole　machine.　Lost　motion　is　the　key　indices　to　characterize　robot　joint　transmission　accuracy　and　performance,　and　its　measurement　and　evaluation　is　an　important　foundation　for　robot　joint　performance　improvement.　The　measurement　level　of　robot　joint　lost　motion　is　still　limited　to　the　measurement　of　single　transmission　device,　such　as　reducer　and　motor.　The　researches　on　measuring　robot　joint　lost　motion　is　still　small.　This　paper　analyzes　the　measuring　principle　and　methods　of　robot　joint　lost　motion,　and　designs　a　testing　machine　for　robot　joint,　which　provides　a　strong　support　for　establishing　the　overall　quality　evaluation　system　for　robot　joint.　The　correctness,　effectiveness　and　practicability　of　the　proposed　method　and　theory　have　been　verified　by　targeted　experiments.　It　shows　that　the　testing　machine　can　meet　the　requirements　of　measuring　lost　motion　of　different　types　of　robot　joints.