In order to ensure the safety of electric slip ring system， it is necessary to carry out the research on the fault diagnosis method of electric slip ring. This method could provide the basis for the fault warning and troubleshooting of single point mooring electric slip ring. In this paper， a fault diagnosis model based on CART decision tree algorithm is adopted to diagnose the single point electric slip ring fault of FPSO. The sample data set is made by finite element method， and the fault diagnosis model of electric slip ring is obtained by training the data set. Finally， the diagnosis model is simplified by the post pruning method， and the fault diagnosis of the electric slip ring is realized. The analysis results show that the model has high accuracy and fast diagnostic speed， and can effectively diagnose the electric slip ring， so that the staff can take measures to repair or replace the electric slip ring in time to ensure the safety and normal operation of the FPSO.