The invention relates to a method for removing old oil and oil sludge in a transformer, which is used for removing old oil and oil sludge in the transformer during replacement of transformer oil in a large-sized electric transformer. The method for removing old oil and oil sludge in the transformer is reliable, complete flushing is realized, and old oil is replaced completely. The dielectric loss is small after replacement of oil in the transformer, the pre-service standard of super-high-pressure oil is reached, the stability is ensured, and rebounding is prevented. Therefore, residual deteriorated oil molecules in the transformed oil are removed, and various indexes reach the national standard after oil replacement. A large amount of production cost can be saved for an enterprise, and energy is saved for the country.