The invention discloses a modular substation OPGW (optical fiber composite overhead ground wire) residual cable rack. The overall weight of the modular substation OPGW residual cable rack is light, each module is small in size, light in weight and convenient for batch production and processing, a residual cable support is installed efficiently and conveniently, transformation and replacement are carried out conveniently, the stock size of the residual rack can be adjusted according to the field and requirements, and the shape of the residual rack can be adjusted according to the field and requirements. The modular substation OPGW residual cable rack is fixed on portal rod through a U-shaped hoops and hoop locking bolts. The modular substation OPGW residual cable rack comprises a residual cable rack mounting frame device, a residual cable support and cable fixing modules, wherein the residual cable rack mounting frame device is fixed on the U-shaped hoops, the residual cable support is made of aluminum alloy and comprises an upper mounting cross beam, a lower mounting cross beam and vertical mounting beams, both the upper mounting beam and the lower mounting beam are provided with multiple groups of mounting holes, a plurality of cable fixing modules are arranged, each cable fixing module is connected with one group of mounting holes of the residual cable support by using mounting bolts through a mounting boss, and each cable fixing module comprises lock bolts used for locking a well stocked optical cable. The invention further discloses an assembling method of the residual cable rack.