Aiming at the problem of that many abnormal conditions may occur in the process of equipment deterioration, a maintenance model based on the three-stage time delay theory is proposed. Firstly, because the process from equipment defect to failure does not obey the same distribution, based on the three-stage time delay model, the health status of the equipment can be classified to original defect, serious defect and failure. Different distribution functions are defined at different stages to simulate the deterioration process of the equipment. Then, the relationship between the time of equipment defect and failure and the threshold time point is analyzed, and the maintenance situation is classified. The model with the lowest maintenance cost per unit time is established to find out the best preventive maintenance cycle time and the optimal threshold time. Finally, the genetic algorithm is used to solve the proposed model. The effectiveness of the model is verified by a case. It is helpful for enterprises to carry out preventive maintenance inspection regularly according to the maintenance plan, and to prevent and repair the initial defect state and serious defect state of the equipment according to different conditions.