This paper analyses the takeoff process of a small scaled UAV (unmanned aerial vehicle) with a single rocket booster. Because the thrust provided by the rocket booster is 10 times as large as the thrust provided by the engines, the effects caused by the boosting rocket on total mass, compound centre of gravity and inertia can not be neglected and are all considered. The inertia of the boosting rocket is calculated by the means of finite element method. Based on the analysis, a nonlinear dynamic model of the UAV is built. Several simulations with different takeoff parameters are conducted to test the takeoff performance. By analyzing simulation results, the acceptable range of boosting angle is investigated.