The invention discloses a novel flange connecting structure which comprises an upper flange neck and a lower flange neck coaxial with the upper flange neck. An upper ISO-K vacuum flange is arranged on the outer wall of the lower end of the upper flange neck. A lower ISO-K vacuum flange is arranged on the outer wall of the lower end of the lower flange neck. The outer wall of the upper flange neck is sleeved with an upper elastic collar, and the outer wall of the lower flange neck is sleeved with a lower elastic collar. An upper loose quick-opening flange plate is arranged on the outer wall of the upper ISO-K vacuum flange, and a lower loose quick-opening flange plate is arranged on the outer wall of the lower ISO-K vacuum flange. A first semicircular clamp and a second semicircular clamp used for locking are arranged on the outer wall of the upper loose quick-opening flange plate and the outer wall of the lower loose quick-opening flange plate correspondingly. One ends of the first clamp and the second clamp are connected through a hinge, and the other ends of the first clamp and the second lamp are clamped though a connecting rod and a nut. According to the novel flange connecting structure, the ISO-K vacuum flanges can be converted into KF flanges without additional arrangement of a transitional tube, the connecting structure is compact in structure, small in occupied space, and capable of facilitating rapid mounting and dismounting of flange connectors.