The　test　bench　of　lost　motion　of　a　precision　reducer　is　a　non-standard　piece　of　equipment，and　there　has　been　no　systematic　research　on　its　precision　characteristic.　Therefore，verification　regulation　for　evaluating　the　metrological　performance　of　such　a　test　bench　is　lacking.　To　evaluate　the　accuracy　characteristic　of　the　test　bench　of　lost　motion　of　a　precision　reducer，an　accuracy　evaluation　method　is　proposed.　First，the　structure　of　the　test　bench　is　analyzed，and　the　main　error　sources　that　affect　the　precision　of　the　test　bench　are　determined.　These　error　sources　include　the　measurement　error　of　the　torque　sensor，measurement　error　of　the　angle　sensor，friction　torque　of　the　torque　measurement　chain，and　elastic　torsion　angle　of　the　angle　measurement　chain.　Second，the　function　mechanisms　of　these　error　sources　are　studied，and　a　method　of　converting　torque　error　into　angle　error　is　proposed　through　a　hysteresis　curve　so　that　all　types　of　error　are　unified　as　an　angle　error.　Then，the　concept　of　local　linearization　of　the　hysteresis　curve　is　proposed　to　realize　the　simple　calculation　of　torque　error　mapped　to　angle　error.　Finally，a　method　of　evaluating　the　accuracy　of　the　test　bench　is　given　according　to　the　mathematical　expression　of　lost　motion.　Testing　was　conducted，and　the　experimental　results　indicate　that　the　test　error　of　geometric　lost　motion　is　±0.　04＇，test　error　of　elastic　lost　motion　is　±0.　06＇，and　test　error　of　total　lost　motion　is　±0.　04＇.　Consequently，the　accuracy　of　the　test　bench　of　lost　motion　meets　the　test　requirements，and　the　method　proposed　is　effective　for　evaluating　the　accuracy　of　the　test　bench.　In　addition，the　test　accuracy　can　be　improved　by　testing　several　places　within　one　rotation　of　the　output　terminal　of　the　precision　reducer.　©　2023　Chinese　Academy　of　Sciences.　All　rights　reserved.