As the core component of industrial robot RV reducer, its health condition determines the life and accuracy of the robot. The main reason that restricts the long-term stable operation of industrial robots. This paper analyzes and compares the advantages and disadvantages of various technologies from three aspects: state detection, signal processing and fault diagnosis. The problems of developing real-time online evaluation are also discussed. Deep data mining and exploring new methods of RV reducer condition monitoring are the research difficulties in this field. It provides a reference for further research in the field of RV reducer.