The invention discloses a heat exchanger suitable for fluid phase change on the outer side. The heat exchanger comprises a left collecting pipe, a right collecting pipe and multiple micro channel flat pipes, wherein the left collecting pipe and the right collecting pipe are parallelly arranged, the multiple micro channel flat pipes are arranged between the left and right collecting pipes at intervals in the length direction of the left collecting pipe or the right collecting pipe. The outer pipe surface of each micro channel flat pipe is provided with metal foam, and the space between two adjacent micro channel flat pipes is also provided with metal foam. The beneficial effects of the heat exchanger are that each micro channel flat pipe is covered with the metal foam, so that the heat exchange capability outside the pipe is enhanced; and the structure of the metal foam is similar with the structure of a steel wire ball, so that a liquid medium cannot exist in the metal foam when flowing through the metal foam, the liquid medium flows down along gaps of the metal foam, and therefore a condition that the operation of the heat exchanger is influenced due to a phenomenon that the medium freezes on the outer surface of the heat exchanger when the medium is converted into a liquid phase from a gas phase can be effectively avoided.