The invention discloses a method for replacing butterfly valves at the lower portion of a transformer, and relates to the technical field of electric-power transformer overhaul technologies. The method includes the following steps that an oil inlet pipe of an oil filter is connected with an oil discharging valve, and an oil outlet pipe is connected with an oil tank; a control valve at an oil outlet of an oil storage cabinet is closed, an oil discharging valve is opened to discharge oil, and when an oil level descends to a position which is a preset distance away from on the top of a transformer body, oil discharging is stopped; a gas relay is removed, one side, adjacent to the oil storage cabinet, of the gas relay is sealed by a side plate, and a connection pipeline of the gas relay is connected to a vacuum pump; all control valves are closed, and the vacuum pump is started to conduct vacuumization; when a reading of a vacuum pump pressure gauge reaches a designated value, the vacuumization is stopped; the butterfly valves at the two sides of an oil-submerged pump are closed and rapidly replaced. The method for replacing the transformer lower butterfly valves is reasonable in design, the vacuumization is conducted on the transformer body by the adoption of the vacuum pump so that under the situation that oil in the transformer is not discharged completely, operation of replacing the lower butterfly valves can be rapidly finished, the power-off time of the transformer is shortened, loss of the transformer oil is avoided, and man power, material resources and financial resources are saved.