The utility model provides a rectangular flange, which aims at solving the technical problem that a sealing groove of the traditional rectangular flange is arranged in the rectangular flange in a shape like a Chinese character 'jing'. The utility model has the defects that: the processing mount is large; the sealing groove is open, so four corners of the sealing groove are not positioned after a rubber ring is heated and expanded; and the contacted area of the rubber ring and the air is large, so the rubber ring is easy to age. The essential points of the utility model are that the sealing groove is rectangularly arranged in the rectangular flange. Four corners on the outside of the sealing groove are arc and the inside of the sealing groove is provided with a driving and reversing knife edge. The utility model has the advantages that a closed sealing rubber strip without an opening is placed in the sealing groove, so the contact of the rubber ring and the air is reduced and the service life of the sealing rubber strip is prolonged.