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 . <EOS>