This study builds a moderated mediation model to empirically investigate the impacts of product modularity and supply chain integration (i.e., supplier, customer, and internal integration) on the flexibility and moderating effects of the product life cycle (i.e., growth and maturity stages) on the relationships using data collected from 204 Chinese manufacturers. The findings reveal that both supplier integration and internal integration mediate the relationship between product modularity and flexibility. Moreover, the indirect effect of product modularity on flexibility through supplier integration is stronger during the growth stage than during the maturity stage. There is no difference in the indirect effect of product modularity on flexibility through internal integration, which is significant in both the growth and maturity stages. In addition, the impact of product modularity on customer integration is significantly stronger, whereas that of customer integration on flexibility is significantly weaker during the growth stage than during the maturity stage.