This paper proposes a new maintenance modeling for evaluating Mean Time To Repair (MTTR) of complex systems based on Goal Orient (GO) method. First, the GO operation formulas of MTTR for the frequently-used GO operators are proposed. Then, the system MTTR evaluation process of this paper' method is formulated. Finally, this paper's method is used to evaluate the MTTR of a hydraulic oil supply system for an armored vehicle firstly. The analysis process shows that this paper's maintenance modeling is closely linked to system structure and more objective, and the system MTTR is obtained by multiple GO operations easily. All in all, this study not only widens the application of GO methodology; but also provides a new maintenance modeling for maintenance design and on-condition maintenance.