A heat exchanger comprising a first collecting pipe; the first collecting pipe comprises a first upper main board and a first lower main board; a first channel and a second channel are formed between the first upper main board and the first lower main board; a plurality of flat tubes extend into the first channel and the second channel, respectively; wherein the first upper main board and the first lower main board are hermetically connected; the heat exchanger further comprises a second collecting pipe which is provided with a third channel and a fourth channel; the flat tubes are separated in two rows; one ends of the flat tubes are communicated with the first channel and the second channel, respectively; and the other ends of the flat tubes are communicated with the third channel and the fourth channel, respectively; and the third channel is communicated with the first channel through a row of the flat tubes, and the fourth channel is communicated with the second channel through another row of the flat tubes; characterized in that the first upper main board has a flat top surface; and in that a vertical height between a highest point of the first channel and the second channel and a lowest point of the first channel and the second channel is L1, a maximum width of each of the first channel and the second channel is L2, and a ratio of L1 to L2 is not greater than 1:4; wherein the one ends of the flat tubes extend into the first channel and the second channel, respectively; and wherein the first lower main board comprises a pair of side walls bent toward the first upper main board and abutting against the first upper main board, and the first lower main board comprises a first middle rib abutting against the first upper main board; and the first channel and the second channel are formed by the first lower main board, the side walls, the first middle rib and the first upper main board. the first upper main board and the first lower main board are hermetically connected; the heat exchanger further comprises a second collecting pipe which is provided with a third channel and a fourth channel; the flat tubes are separated in two rows; one ends of the flat tubes are communicated with the first channel and the second channel, respectively; and the other ends of the flat tubes are communicated with the third channel and the fourth channel, respectively; and the third channel is communicated with the first channel through a row of the flat tubes, and the fourth channel is communicated with the second channel through another row of the flat tubes; characterized in that the first upper main board has a flat top surface; and in that a vertical height between a highest point of the first channel and the second channel and a lowest point of the first channel and the second channel is L1, a maximum width of each of the first channel and the second channel is L2, and a ratio of L1 to L2 is not greater than 1:4; wherein the one ends of the flat tubes extend into the first channel and the second channel, respectively; and wherein the first lower main board comprises a pair of side walls bent toward the first upper main board and abutting against the first upper main board, and the first lower main board comprises a first middle rib abutting against the first upper main board; and the first channel and the second channel are formed by the first lower main board, the side walls, the first middle rib and the first upper main board.