Harmonic reducer is one of the important parts in industrial robots. Its performance is directly related to the working states of industrial robots. The fatigue failure of the harmonic reducer is divided into two types: flexspline failure and flexible bearing failure. A comprehensive test platform for harmonic reducer was used to monitor and record signals of the transmission efficiency, temperature and vibration of the harmonic reducer in real time, then two kinds of failure modes were detected by recognizing the differences among the signal characteristics. The relationship among the signals of transmission efficiency, temperature and vibration caused by the failure of the harmonic reducer was evaluated. In the case of the flexspline failure, the transmission efficiency degradation of the harmonic gear drive is about 7%; in the case of the flexible bearing failure, the transmission efficiency degradation of the harmonic gear drive is about 15%. The analysis of the failure mode characteristics for the harmonic gear drive is of great significance for the study of failure and fatigue.