Owing to the coupling of position and attitude control， a conventional under-actuated quadrotor gun-armed unmanned aerial vehicle （UAV） is incapable of hovering fire with pitch angles. Besides， the fire control strategy is complicated during the mobile fire process. In this paper， a fully-actuated tilting quadrotor gun-armed UAV is designed as a gun platform， by which not only the system is decoupled but also the control allocation method is simplified. First， the system is modeled by the Newton-Euler method mathematically. Then， a controller is designed based on the backstepping approach， and the stability of the system is verified. Finally， simulation experiments are conducted with the Simulink software. The experimental results demonstrate that the fully-actuated tilting quadrotor gun-armed UAV model designed in this study can achieve decoupled control of position and attitude with a minimal tracking error. It can meet the requirements of hovering and mobile fire missions， and is suitable for flight/fire control systems with high real-time demands.