Abstract: A new method for measuring the angular backlash of a reducer of the involute planetary drive system with small tooth number difference was put forward, and a set of measuring equipments was designed and made accordingly. It can be used to measure the angular backlash of a reducer both in simulated environment such as high and low temperature, vibration, acceleration, centrifugation, moisture, etc and in real load condition. After measuring in a reducer, the results showed that the new way was feasible and the performance of the equipments was reliable and excellent.