To analyze the process of rocket booster launching of unmanned aerial vehicle （UAV）, an UAV nonlinear mathematical model and the flight control system are established aiming at the big flight influences of the additional moment acting on the body caused by the installation position error of two rockets and the time of spreading wings. The boundary value of installation position error and the appropriate spreading time are determined by using the variably method. The simulation results show that the obtained boundary value and the time meet the requirements of actual flight.