Abstract: The modular design and construction approach is applied to AP1000 nuclear power project, which can significantly shorten the construction period in theory, and this is the first application in the field of nuclear power construction in China. Due to the influence of the first of a kind project, the actual construction period of AP1000 is far beyond the theoretical construction period. For the internal structure of the reactor building which is on the critical path, if the structural module is used, the shortest construction period is 19 months on the condition that risk factors are excluded; if the ordinary reinforced concrete structure is adopted, the construction period is 12.85 months and 17.68 months（with different reinforcement ration） calculated by the construction efficiency method, and 18.1 months evaluated by expert method, which are shorter than that of the structural module. After analysis, the cause is due to the characteristics of the structural module of AP1000 with more staggered floors and the technical difficulties of modular construction（the connection between the floor module and the wall module is more difficult, and anti-deformation control of the structural module is also difficult）. In the future, if we want to optimize the construction period of nuclear power plant, we need to choose reasonable modular technology, improve the modular design scheme, and simplify the system as much as possible and reduce the engineering quantity.