The invention relates to the technical field of inservice inspection, in particular to a flange supporting device of automatic reactor pressure vessel inspection equipment. The flange supporting device comprises flange switching components, four guide columns and four guide legs; the flange switching components are spliced to be a ring seat; the four guide columns are fixedly located at the upperends of the flange switching components; the guide columns quarter the ring seat; and the four guide legs are fixedly located at the lower ends of the flange switching components. Through the use of the flange supporting device, the situation that the automatic reactor pressure vessel inspection equipment shields a threaded hole and a sealing groove is avoided, and the effects of shortening the unit overhaul time and improving economic benefits are achieved.