The invention relates to a large oil power transformer maintenance process in a power system, in particular to a method for replacing a butterfly valve of a transformer through oil-carrying vacuumizing. The method is reasonable in design, and a vacuum pump is used for vacuumizing, so that replacement work of the butterfly valve can be completed while the transformer does not release oil completely, time for the whole replacement process is saved, outage time can be reduced effectively, power sale quantity is increased, economic loss is lowered, working efficiency is improved, waste of manpower resources is reduced, manpower resource expense is saved, and transport expense caused by transporting of large equipment is reduced.