The invention relates to a mounting method for an insulating layer of a nuclear reactor pressure vessel. In order to optimize the whole nuclear island mounting construction period, in the mounting method, a connecting ring, a hanging bracket, a channel steel ring, a barrel insulated board, and the like are assembled into a prefabricated whole, and the prefabricated whole is hung on a reactor shaft; then a pressure vessel is hung; accessories in the process of mounting are disassembled, and proper adjustment is carried out, wherein when the pressure vessel is arranged, the mounting work of a barrel insulating layer can be finished in one day, and compared with the time of nine days in a scheduled plan of the prior art, the mounting method saves eight days. In the mounting method, the hanging ring can avoid being operated with heavy load repeatedly for a long time, thereby reducing the probability of ring hanging accidents. The pressure vessel and the insulating layer also can avoid being hung in and out of the reactor shaft for many times, thereby reducing the probability of damage of the outer surface of the insulating layer.