The invention provides a method for expanding a transformer substation transformer oil pool. The transformer substation transformer oil pool can be expanded with the shortest construction period and the lowest cost. The method includes the following steps that 1, an oil pool bottom plate and outer ground affecting oil pool wall movement are opened, and a base slot is excavated to the designed depth at new oil pool wall design positions; 2, corner pool wall components of the original oil pool are expanded outwards and moved to the corresponding positions of a new oil pool; 3, main pool walls of the original oil pool are expanded outwards and moved to the corresponding positions of the new oil pool; 4, four 0.5m additive pool wall components are added to the new oil pool; 5, two 1.0m additive pool wall components are added to the new oil pool; 6, bituminous mortar is adopted in oil pool seams for seam pouring, and silicone weather-proof glue with the thickness of 10 mm is adopted in the surface of the oil pool for sealing; 7, a high-capacity transformer is installed on a guide rail, the oil pool bottom and outer ground are restored according to the requirements of the original oil pool, and iron gratings and cobbles are laid. The oil pool can be rapidly expanded, the construction period is short, and cost is low.