Aiming at the initial disturbance caused by the clearance between the shelves during the launching of rocket and missiles， a method combining random disturbance excitation with multi-body dynamics was proposed. Based on the dynamics of multi-body system， the launch dynamic model modeling the interaction between missile and launcher was established. The Hertzian collision contact algorithm and stochastic overload model were coupled to the multi-body dynamics model. Through the simulation analysis of missile with launcher， the results show that the coupled model can effectively capture the influence of the clearance of the missile frame on the missile launching disturbance. The larger the clearance between the missile body support and launcher， the larger the random disturbance force produced during the missile launching process （before derailment）， thus leading to large attitude deviations of the projectile in Y and Z directions after derailmentand also a big dispersion effect of the projectile after launching derailment. This method can provide an effective reference for the study of the impact of the clearance of the missile rack on the launch disturbance， and also for the analysis of the initial disturbance of similar missiles.