The flight mechanical model of tilt-rotor uav is the basis of designing flight control rate and analyzing flight characteristics. Based on the engineering reality, the nonlinear flight mechanics model of a new configuration tilt-rotor uav was set up. Features and advantages of the new type rotorcraft are analyzed compared with the traditional tilt-rotor. The flight dynamics model of helicopter model, conversion mode and airplane mode was developed. Based on the flight mechanics model, taking helicopter mode for example, trim and stability were analyzed for different flight speed. The calculate results show that, this configuration tilting rotor has good lateral and directional stability in helicopter mode; The lateral and longitudinal coupling of helicopter hover mode is very small.