Basic combat unit (BCU) is the minimal unit that can independently execute military missions. It is usually composed of one set of weapon system, including many weapons and equipment. This paper takes basic combat unit as a whole to study, analyses and defines its state based on its mission reliability function. In addition, a mathematic model of availability is set up through Markov stochastic process. At last, some numerical examples are given to illustrate the practicability of the model in the paper.