A heat exchanger comprising: a mixing and redistribution header (20) at one end of the heat exchanger; multiple heat exchange tubes (30) in communication with the mixing and redistribution header (20). An upper cavity (21) and a lower cavity (22) in communication with each other are disposed in the mixing and redistribution header (20); a fluid entering the heat exchanger first of all flows into a part of the lower cavity (22) of the mixing and redistribution header (20), then is collected and mixed in the upper cavity (21) of the mixing and redistribution header (20), and is distributed into another part of the lower cavity (22) and flows out through a heat exchange tube (30) in communication with the lower cavity, a cross-sectional area of the upper cavity (21) being equal to or greater than a cross-sectional area of the lower cavity (22).