For a repairable system consisting of multiple equipments of the same type, spare demand and its supply plan will affect the number of workable equipments, and hence exerts effects on the demand of other spares for the equipment. Therefore, the assumption that the spare demands are mutually independent and have no relation with the spare supply plan of the system is not correct. This study considers the correlated demands as well as the effects of the spare supply plan on the spare demand, and describes the spare demand law by the Markov process. Availability calculus methods are presented according to the two cases of cannibalization and non-cannibalization respectively. Based on the methods, a spare optimal allocation model is constructed by setting spare allocation cost as the optimization target and system availability as the constraints. The model is solved by marginal analysis. Finally, a case study is presented to illustrate the validity of the proposed model.