A heat exchanger includes a first manifold, a second manifold, a plurality of flat tubes, and a plurality of fins. Two ends of the first manifold are respectively sealed with a cap. The heat exchanger further includes a first connecting pipe, a second connecting pipe, and a third connecting pipe. The first connecting pipe communicates with the first manifold via a second opening, the second connecting pipe communicates with an allocation tube, and the third connecting pipe communicates with the second manifold. A diameter of the first connecting pipe is greater than the diameter of the allocation tube. The two connecting pipes of the heat exchanger correspond to refrigerant in different states. The diameters of the two connecting pipes are different such that the refrigerant in different states may be uniformly allocated, which contributes to the efficiency of the heat exchanger.