A connecting structure, including: a volute including an air inlet and an air outlet; and a conversion interface. A connecting part is disposed on an outer side of the air outlet. The connecting part includes: a lower plate; a lower lug protruding outward from each end of the lower plate; and an upper flange disposed above the lower plate. The conversion interface includes a sleeve. An upper plate protrudes from a rear part of the sleeve. Two upper lugs protrude outward from two ends of the upper plate, respectively. A lower flange is disposed on the rear part of the sleeve beneath the upper plate. The upper plate is disposed on the upper flange. The lower flange is disposed on a curved convex block that protrudes upward from an outer edge of the lower plate. The upper lug is disposed on the lower lug.