Aiming at the problem of lacking of effective method in the determination of aircraft testability index,a new method is proposed based on Generalized Stochastic Petri Nets. Through analyzing the maintenance process of the aircraft line level,the GSPN model of the process is constructed. Then the model is solved and analyzed. Meantime the relationship between testability index and operational availability and mean time to repair is also obtained. Using the two indexes as a constraint,rational testability index is determined by trade-off analysis. Finally,taking the determination of one aircraft testability index as an example,the feasibility of the method is demonstrated.