The invention discloses a method for mounting a main return loop of a nuclear power station pressurized water reactor coolant system, which is characterized in that: a main pipeline heat section bend (16) and a main pipeline transition section bend (17) are welded at a loop nozzle of a lower seal end of the used steam generator (1); during mounting, a welding line (9) between a main pipeline heat section (13) and the main pipeline heat section bend (16) on the lower part of the steam generator (11) and a welding line (2) between the main pipeline heat section (13) and a reactor pressure container (10) can be assembled and welded simultaneously. In addition, the working amount for a welding line (5) is reduced during mounting. The mounting method can better control and guarantee the assembling and welding quality, improve the working efficiency, reduce the mounting time, and shorten the construction period.