Abstract: The test data of large complex products presents small sample characteristic in each phase during the development process normally. The restriction, the reliability of one phase is always higher than that of the previous one, was recommended to be used for reliability evaluation based on multi-phase test data. The reliability of product is increasing during the development process normally, therefore, such method has broad applicability. The reliability was considered a random variable with such restriction. The reliability probability distribution function based on the restriction was given, and an approximative calculational method was presented. An example was given to compare the result obtained from single phase test data with that based on multi-phase test data. Since the precondition is brief and the reliability probability distribution function can be achieved, the mentioned method is more applicable to practical applications in comparison with traditional army materiel system analysis activity(AMSAA)-model, and it can gain the reliability interval estimation which can not be obtained from Duane-model.