In the process of the equipments used, operational availability (Ao) is the common top-level parameter for the operational readiness. However, current Ao’s theories and methods toward support objects can mainly be applied to low levels which are lower than basical equipment combat unit, single equipment, but is inapplicable with higher levels such as equipment combat unit and materiel system. Firstly, the paper introduce the concept and usage mission model of the smallest mission unit(SMU). Then research the concept and calculate method of the SMU's operational availability. And research the status update model and simulation flow of accumulation workload time of the SMU in the Joint Operation Equipment Support Simulation and Evaluation System(JOESSES). And simulation calculate of the SMU's Ao. Finally , in the JOESSES ,the method which be created in this paper be proved to be credible and availability.