As a new configuration tilt rotor aircraft， the variable diameter tiltrotor aircraft can provide design basis for the selection of variable diameter plan in the design stage according to the simulation results of its flight characteristics. In this paper， the flight performance calculation model and nonlinear flight dynamics model of variable diameter tiltrotor aircraft are established， and the prototype is simulated. According to the parameter influence on flight performance and trim results， the rotor diameter variation range， diameter variation timing and diameter variation control strategy of the numerical example prototype are planned. Finally， the best rotor diameter variation range of the prototype is 0.7R—1.15R. The best time to change the diameter is to complete the process of rotor diameter increase when flying forward in 360 km/h fixed wing mode， and complete the process of rotor diameter decrease when flying forward in 250 km/h fixed wing mode. At the same time， a variable cross-section twist tube type variable diameter rotor control strategy is designed based on Sikovsky variable diameter rotor.