The invention discloses a single-row variable pipe diameter heat exchanger, comprising a left side pipe plate, a right side pipe plate, fins, U-shaped heat exchange pipes, an elbow and a connecting pipe, wherein, the U-shaped heat exchange pipes penetrate through the preset holes on the left side pipe plate, fins and the right side pipe plate to be expanded and fixed with the left side pipe plate, the fins and the right side pipe plate; the pipe diameters of the U-shaped heat exchange pipes are more than two kinds; the U-shaped heat exchange pipes are distributed in a single-row mode or partially distributed in a double-row mode; the fins are in a sing-row integral type; the apertures of the preset holes on the fins are more than two kinds; the U-shaped heat exchange pipes are distributed in segments; the pipe diameters of the U-shaped heat exchange pipes are the same within each segment range; the number of the U-shaped heat exchange pipes is 2-18 within each segment range; the large-pipe-diameter U-shaped heat exchange pipes are distributed in a single-row mode; and the middle-pipe-diameter U-shaped heat exchange pipes are distributed in a single-row mode or double-row mode. The heat exchanger provided by the invention has the characteristics of simple and reasonable structure, small volume, low manufacturing cost, high heat exchange efficiency and wide application range and the like, and is flexible in operation.