A flanged connecting assembly, which comprises: a first flange (210); a second flange (220) with a diameter greater than the diameter of the first flange; and a third flange (230), provided with a first portion (231) and a second portion (232) forming a step; wherein the first portion (231) is fitted to the first flange (210) through a first sealing element (310), and the second portion (232) is fitted to the second flange (220) through a second sealing element (320); and the first flange (210) and the second flange (220) are spaced from each other by a first distance. In this way, a gap formed under the fit of the three flanges can allow a used sealing element to be taken out vie the gap and/or a new sealing element to be assembled in via the gap.