Abstract: By considering the influence of rotor wake on wings and horizontal as well vertical stabilizers, and by combining the blade-element theory and momentum theory, a model for a tilt-rotor aircraft with slung load is developed. Based on the model, taking an XV-15 tilt-rotor aircraft as an instance, the trim results of straight forward flight under different airspeed and flight modes are obtained with the Levenberg-Marquardt method on MATLAB platform. And compared them with NASA's reference data, the model is proved to be valid. The influence of slung loads on the control strategy and Euler angle is also analyzed. The results show that:under a helicopter mode, the slung load influences deeply the collective pitch, and its influence on pitch angle attenuates as the increase of velocity, which displays a difference with traditional helicopter; under a conversion mode, the slung load can effectively slow down the decrease of the pitch angle as well as longitudinal stick; under an airplane mode, the flight with slung loads is not suggested.