To solve the problem of the harsh and complex working environment, high system reliability, difficult maintenance and high maintenance cost of the electro-hydraulic composite underwater tree during operation, the reliability degradation model of the underwater Christmas tree components based on the gamma degradation process and the maintenance cost model that takes into account the maintenance costs and production loss are established, and the objective system of preventive maintenance optimization for the underwater Christmas tree system is established. The model is optimized based on multi-objective particle swarm optimization algorithm. The model established can effectively describe the working characteristics of the underwater Christmas tree system. The optimal solution can ensure the system is highly reliable under the premise of low maintenance costs.