The invention provides a method for installing a concealed installation distribution box in a block wall and an applied fixing device. The elevation and the horizontal size are determined in a reserved hole, when the distribution box is installed, a box body of the distribution box is fixed through fixing plates and fasteners, wiring is finished, workers majoring in civil engineering continue to carry out construction, tamping and screeding are carried out on the two sides of the distribution box, the fixing device is dismounted, edge sealing is carried out, then surface layer construction and finished product protection are carried out, and the dismounted fixing device can be recycled and cannot be wasted. According to the method for installing the concealed installation distribution box, labor cost and cost of civil engineering materials are saved, later reworking caused by the quality problem is avoided, manpower, material resources and time consumed by reworking are saved, construction efficiency is improved, and the functions of saving energy and reducing consumption are achieved. The method has the advantages that the technological economic benefits and social benefits are obvious, and the method has application and popularization prospects in the concealed installation distribution box installation through BM lightweight aggregate chain building blocks.