The tiltrotor Unmanned Aerial Vehicle (UAV) attracts increasing concern due to the ability of vertical take-off and land and high-speed cruise. In this study, a tiltrotor UAV with three rotors is built. A dynamic inversion based control system adopting time-scale separation design is developed for the proposed tiltrotor UAV simulation model in rotor flight mode. The outer loop of controller realizes speed tracking and the inner loop of controller realizes the attitude stable. Design the control allocation for the nonaffine system of the proposed tiltrotor UAV. Simulation results demonstrate the proposed flight controller could realize speed tracking and attitude stable whether with disturbance or not.