The modeling & simulation of the flying performance of helicopters is developed here in order to maintain them based on their states. It is centered on the modeling of the working characteristics of the turboshaft. First, the descriptions of the flying performance of helicopters and the characteristics of the turboshaft are discussed, and a math model of the working process of the turboshaft is built. Then the model is predigested according to the calculating program of the engine's test performance. Finally, the flying performance simulation is realized through the calculation based on the simplified model. The results show that this method is credible and high efficienct.