The invention relates to retaining walls, firewalls, accident oil pool thin walls and other structures of transformer substations, in particular to a transformer substation cast-in-place wall combined steel die and a construction method of the transformer substation cast-in-place wall combined steel die. The combined steel die comprises at least one steel standard face arranged on an inner wall face and an outer wall face of a cast-in-place wall and at least one steel non-standard face arranged on the end face of the cast-in-place wall, each standard face is formed by arranging an additional layer (4), a standard layer (3), an additional layer (4) and a standard layer (3) from bottom to top in sequence in a staggered mode, the height of the additional layers (4) is 0.2-0.3m, the standard layers (3) are mainly formed by connecting standard pieces (1), the length of the standard pieces (1) is below 3.5m, and the height of the standard pieces (1) is below 1.5m. According to the transformer substation cast-in-place wall combined steel die and the construction method, the quality problems that a die plate is easy to swell and pulp leakage happens to a seam position due to a traditional construction method are solved, meanwhile, scaffold erection and other complex processes are reduced, labor intensity is lowered, and the construction speed is improved to a large extent.