The utility model relates to an automatic regulator for transformer capacity, which is composed of a high voltage auxiliary switch, a low voltage auxiliary switch, a current mutual inductor, etc. The utility model is characterized in that the high and low voltage auxiliary switches are respectively arranged on a high voltage inlet wire terminal and a low voltage outlet terminal of a transformer, which is suitable for the parallel operation mode, etc. with two or more transformers; when load is reduced to a setting value, superfluous transformers can be quit; when the load is enlarged to a setting value, the quit transformers can be put into operation; the purposes of the utility model is to reduce the no-load loss of transformers. Under the condition of definite capacity and compared with the mode of adopting a large capacity transformer which is in an isolated operation, the utility model has the advantages of energy saving, no power failure during the examination and repair of transformers, etc.