A low-cost small solid launcher using both lateral jets and gird fins for its attitude control purpose was designed. According to the layout of the vehicle, this paper designed a cold separation scenario to which a close-loop attitude control scheme is introduced. Based on the description of the separation process, dynamic and kinematic equations were built. Meanwhile the ejector piston model, retro-rocket model, aerodynamic model and attitude control model were also established. The collision detection method was discussed and Monte Carlo simulation was introduced to deal with the uncertainties. Simulation results show that the proposed project can not only reduce the attitude deflection of the vehicle but also avoid interstage collision, so it provides a possible choice for the stage separation of this type of vehicles.