Aimed at the demand of spare parts changes dynamically as the mission phases change, and considering the influence of the scrap probability owing to the finite carrying repair capacity of combat units, by the introduction of the scrap factor, the evaluation model of time-varying operation availability for two echelon support system was developed, in which, the failure parts can be scrapped and there is no exterior supply. And then, a simulation is realized based on the ExtendSim software and the availability results was received. According to the results, the parameter fitting was carried into execution and the approximate analytical formula of scrap factor in the availability model was obtained. In a given example, the scrap factor can adapt to different reliability and maintainability parameters, and the adaptability is validated. The evaluation model is a complementary for conventional models which is developed based on the spare parts balance in support system and can help equipment personals to make appropriate spare parts programs. 