The implementation of combat unit is taken as background when the unit is far away from the support base. Considering that repair capacity of combat units is limited, the approximation evaluation model of availability is developed based on the reliability theory when the failure parts can be repaired partially for two echelon support systems. Through the Monte-Carlo simulation model, the model is validated and the proposed method is proved to be accurate to a certain extent. When the spare parts cost is taken as object and the equipment availability as restriction, the spare parts optimization model with finite repair capacity is proposed. The margin optimization method is used to realize the spare parts configuration. In a given example, the optimization model and method can help realize rational spare parts configuration, and they would help equipment personals make decisions in spare parts project design and availability evaluation.