In order to accurately analyze the thrust line off-set size of solid rocket motor at the design stage, a simulation calculation method based on Monte-Carlo was put forward. Through the analysis of the factors affecting the thrust line off-set, the mathematic model of transverse size and orientation based on polar coordinates system was carried out, the distribution of each influence factor was discussed, and Monte-Carlo method was adopted for simulation. The result shows the above method is easy and fast to compute thrust line off-set, which is referable to optimize design parameter and estimate design result.