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.