Abstract: For a certain third-generation PWR NPP with world advanced level, its reactor is a facility which used to maintain controlled, self-sustaining nuclear fission chain reaction and realize nuclear energy utilization, while the reactor internals is a key component to ensure that the reactor can maintain controlled fission reaction. The reactor internals features complex installation process, considerable technical difficulties and high installation accuracy. As one of the results of the major national science and technology project, the installation process of reactor internals is improved according to the structural features, fundamental functions and installation technical requirements of reactor intervals of third-generation PWR, thereby simplifying the installation process, enhancing installation accuracy and construction effectiveness, shortening the construction period, reducing construction costs and providing good reference for the installation of internals of follow-up large advanced third-generation PWRs.