To assess the risk of the rocket trail cover separation movement, the multi-body dynamic model in separation process was established according to the Kane's equation, and the collision checking model based on the relative location estimation of the key point-collision boundary was built. The Monte Carlo method and the Morris screening method were used to analysis the uncertainty of the separation process when the input parameters had random deviations. The dispersion characteristics of the separation motion state were got from the Monte Carlo method and the sensitivity of each parameter was ranked by using the Morris screening method. Results show that the trail cover separation solutions possess high reliability, which can satisfy the security requirements in the case of the input parameters with deviations. Among all the input parameters, wind interference and the thrust deviation of the thruster from ejection pistons has greatly influenced the distance of drop points, and the horizontal distance between the rocket and the trail cover has been greatly affected by the wind and the out canister velocity under the circumstances of ignition.