The invention relates to the technical field of sealing structures, and particularly relates to a testing apparatus for stress relaxation of a metal sealing ring flange connecting structure. In the testing apparatus, a lower flange is hermetically installed on a pedestal with a cylindrical inner wall in a matched manner, an upper end face of the lower flange is provided with an annular groove, a metal sealing ring test sample is placed in the groove, and an upper flange located on the metal sealing ring test sample is hermetically on the cylindrical inner wall of the pedestal in a matched manner; a load sensor and a measuring system thereof are placed at the central position of the upper flange, a gland, which is parallel to the upper flange and the lower flange, is arranged on the load sensor and the measuring system thereof, and the metal sealing ring test sample is compressed and fixed through a plurality of bolts which sequentially penetrate the lower flange, the upper flange and the gland and nuts. The testing apparatus can detect the real-time leakage rate in the stress relaxation process of a test sample of a sealing structure by using a vacuum helium mass spectrum leak detection method according to requirements, thereby providing a basis for analyzing the sealing performance of the sealing structure in the stress relaxation process.