Transmission influences on the performance and the security of tilt-rotor directly. A transmission-wing coupling dynamic model including time-varying mesh stiffness and static transmission error was proposed, then calculated the dynamic loads of the transmission-wing coupling system under two flight modes: aircraft and helicopter. Then, computed the life of transmission under different system reliability based on the dynamic loads. This paper presents a more accurate transmission life estimation method. The results present some useful informations for dynamic optimization of the transmission and provide a theoretical basis for the transmission design.