Top-drive system (TDS) is a complex structure of mechanical, electrical and hydraulic integration drilling equipment, which is a high-tech equipment in modern drilling equipment. However, TDS in the process of drilling operation will inevitably have faults, which has high requirements for maintenance personnel. Shortening maintenance cycle, reducing cost and improving operation efficiency have become an important problem for engineering and technical personnel. In this paper, a new fault diagnosis method is proposed based on the current harmonic characteristics to diagnose the deterioration state of equipment. According to the deterioration degree, a prognotics and health management system (PHM) is designed based on the concept of predictive maintenance (PdM), which can truly prevent the failure before it occurs. The designed PHM has been applied to the top-drive device and achieved preliminary results, which lays a foundation for further research and can support the realization of predictive maintenance and health management of the top-drive device.