The reinforced panel was chosen as the object in order to improve the assembly quality of automobile B-pillar, The coordinate values of locator blocks on reinforced panel were taken as optimization variables. The assembly gap between auto body outer panel and reinforced panel was taken as optimization objective. The quadratic response surface models were constructed based on response surface methodology（RSM) and tolerance analysis software three dimensional control system（3DCS). The mutual influence among these different coordinates were analyzed through 3D response surfaces and the 2D contour plots. The multi-objective function was optimized by DesignExpert software. The best combination of coordinate values of locator blocks were obtained, i.e. L1x=1 360 mm, L2x=1 538 mm, L3z=241 mm, L4z=250 mm. Results show that the assembly gaps (d0,d1 and d2) were decreased by 41.5%, 31.8% and 39.7% respectively compared with the initial assembly results. The superb rate was markedly reduced, which effectively solved the problem of excessive gap in the traditional assembly process of B pillar. The production practice further proves the feasibility of the method, which can greatly improve the assembly efficiency.