The invention provides a mounting method for an inner leaf formwork of an assembly type prefabricated outer wall cast-in-place section. The mounting method comprises the steps that (1) sleeves are mounted on the inner side surface of a prefabricated fixing structure and internally provided with inner threads; (2) bolt holes are formed in the inner leaf formwork; and (3) a thread rod is used for penetrating through each bolt hole of the inner leaf formwork, the inner ends of the thread rods are fixedly connected to the sleeves, nuts are arranged at the outer ends of the thread rods, and the nuts are screwed to lock the inner leaf formwork; when the width of the part, exceeding an inner leaf, of an outer leaf is smaller than 150, the sleeves are fixedly connected to the inner side surface ofa pre-embedded steel bar frame of the inner leaf; when the width of the part, exceeding the inner leaf, of the outer leaf is larger than 150, heat-preservation layer connecting pieces embedded in theouter leaf extend to the arranged threads at the ends within the inner leaf range, and the sleeves are fixedly mounted at the ends of the heat-preservation layer connecting pieces; and when the widthof the part, exceeding the inner leaf, of the outer leaf is larger than 250, the sleeves and the thread rods are arranged longitudinally in two lines. According to the mounting method for the inner leaf formwork of the assembly type prefabricated outer wall cast-in-place section, during mounting of the inner leaf formwork, bolt holes do not need to be reserved in the outer leaf, and the outer leaf is not perforated, so that damage to an outer leaf wall body is avoided, the integrality of the outer leaf is guaranteed, and waterproof performance is guaranteed.