The rocket is in unpowered flight at the separation phase of missile and rocket of air launch system and the trajectory is prone to interference from the upper level gust. In order to research the responses of discrete gust of air launch system at separation phase, the influence of gust is simulated by using the grid velocity method, the finite volume method is used to solve unsteady Euler and NS equation, and the separation process is calculated by using CFD dynamic grid. The simulation platform is built by the modular method, including update module of rocket’s status and grid, CFD calculation module of force and moment, solution of flight status parameters and discrete gust interference module. The separation process under 1cos gust disturbance is calculated by using the simulation platform, and the aerodynamic characteristics of the rocket and the carrier with or without gust disturbance are compared and analyzed. The results show that the air launch system can satisfy the requirement of rocket ignition after the separation of missile and rocket in the absence of gusts. After a discrete gust of wind, stability of the air launch system is destroyed after encountering discrete gust and may cause launch to fail, raising a higher requirement for the design of the control rate.