For sharing modules selection in the modular platform design, an algorithm based on constraints was proposed, which was inspired by Collaborative Optimization using the constraints to coordinate the System Level Optimization and Sub-system Level Optimization. In this algorithm, NSGA-Ⅱ was used in the System level to solve the Multidisciplinary Design Optimization problem, and a Pareto set was gotten. By using the fuzzy set theory to evaluate each equation in the Pareto set, the optimum solution was easily picked out, and the sharing modules selection was realized in this way. At last, this method was verified by using an application example constituted by involving a SUV, a hatchback, and a sedan.