［Purposes］ The paper aims to effectively control the cost of prefabricated buildings and promote the rapid development of prefabricated buildings. ［Methods］ Taking the cost control in the design phase as the starting point, combined with the characteristics of complexity, nonlinear and many non-quantitative factors for the cost control of prefabricated buildings, the system dynamics software was used to simulate and analyze the influencing factors of the cost of prefabricated buildings, and the influence of the design phase on the cost of prefabricated buildings was discussed.［Findings］ Strengthening the training of cost control knowledge for designers, improving design standardization degree, the level of design refinement and the collaboration degree among various professions can help reduce the cost of prefabricated buildings. ［Conclusions］ Developers should pay attention to the important role of the prefabricated building design phase and implement effective cost control.