AC contactors are widely used in control systems. An efficient maintenance strategy is the premise to ensure the safe and reliable operation of the system. Due to the inevitable differences of three-phase contacts of AC contactors, the failure threshold of each phase contact is random. However, the existing maintenance decision-making methods do not consider the randomness of the failure threshold, which are not suitable for AC contactors. To solve this problem, this article uses the cumulative arcing Joule integral to characterize the performance state of each phase contact of AC contactor and formulates a performance degradation model considering the randomness of failure threshold, competing failure, and degradation correlation of three-phase contact. A method for estimating the parameters of the degradation model, updating the distribution of the three-phase contact failure threshold and residual life based on maximum likelihood estimation and conditional probability formula, is proposed. A predictive maintenance strategy considering the randomness of the AC contactor failure threshold and the update of the maintenance strategy is proposed. The predictive maintenance strategy takes minimizing the expected cost rate as the optimization goal and adaptively optimizes the preventive replacement threshold of AC contactor three-phase contacts at each predictive maintenance strategy updating time. Finally, the effectiveness of the proposed maintenance strategy is evaluated by case analysis. The results show that, under the predictive maintenance strategy, the expected cost rate of AC contactors can be approximately divided into fluctuation stage, stable stage, and rising stage. The actual cost rate is reduced by about 10% compared with the existing maintenance strategy, indicating that the strategy can make more efficient use of AC contactors and has a better economic effect.