This study sets up a differentiated duopoly model considering capacity constraints and shared manufacturing, investigates the equilibrium results, examines the effects of product differentiation and capacity constraints in three scenarios, and compares the equilibrium outcomes in three cases under Cournot and Stackelberg competition. We find that capacity constraints affect the relationships among product differentiation, equilibrium results, and the market share of enterprises. Shared manufacturing impacts the degree of excess capacity, profits, consumer surplus, and social welfare; however, it may sometimes play a negative role in alleviating excess capacity. Moreover, Cournot competition is a better choice for enterprises with capacity constraints compared to Stackelberg competition.