In order to describe the maintenance activities for the phased array radar′s antenna system, a time based maintenance model that “T/R unit can not be restored to as good as new” under the foundation of reliability was built. Firstly, the antenna′s construction parameters under the T/R unit failure in series were analyzed and the threshold value of failure T/R unit was determined. Then in view of the fact that“T/R unit can not be restored as good as new”, a maintenance model that minimizes the average cost per unit time under the restriction of antenna′s operational availability was established by introducing failure rate increasing factor. Finally, the marginal efficiency analysis algorithm was applied to achieve the optimized maintenance cycle and service group. Several examples are given to verify that the model excludes the restriction of “T/R unit cannot be restored to as good as new”, which are more in accordance with the actual situation and will provide the theoretical foundation for preventive maintenance policy.