Electric power equipments suffer from aging, wearing, weather conditions in operation, the failure rate has a cumulative effect over time. The different grades of preventive maintenance can delay the deterioration of reliability distinctively. Based on the time cumulative effect of the failure rate and the effective age mode considering the reliability improvement with preventive maintenance, and taking the average unavailability as a link, the analytical expressions for the system reliability and economy indices are established. In view of the three kinds of maintenance decision-making targets, such as the system reliability optimization, the minimum of system maintenance cost, and the minimum of system maintenance cost and the system maintenance cost, this paper proposes an algorithm to coordinate and optimize preventive maintenance cycles based on the sensitivity and the differential. Finally, according to the studies on RBTS and IEEE-RTS79, the validity of the algorithm is verified, and it’s analyzed and compared with other different preventive maintenance modes from the perspective of reliability and economy.