By combining decomposition topology model of iterative design for mechanical products with modularization, a modular design method for mechanism based on function-principle-behavior-structure is put forward. Product iterative design decomposition principle based on function-principle-behavior-structure is mainly discussed. Portrait iterative process of function(principle(behavior(structure(function evolvement during design decomposition is analyzed. Transverse dynamic modeling means of product design decomposition is clarrified. Basic modular design method for complicated mechanism based on FPBS is established by introducing modulization into the product iterative design composition process.