The invention relates to a double-pipe heat exchanger. The double-pipe heat exchanger comprises heat exchange sleeve with a trough curved section and a water drainage assembly, wherein the trough curved section is bent in the direction away from the first central axis; the first central axis intersects with the direction of gravity; and the water draining assembly is arranged in the trough curvedsection and communicates with the heat exchange sleeve. According to the double-pipe heat exchanger, water accumulated in the trough curved section is drained through the water drainage assembly, water accumulation, icing and pipe breakage due to expansion in the low temperature environment are avoided, the situation that a refrigerant is mixed with water flow after an inner pipe of the heat exchange sleeve is broken is prevented, and unnecessary breakdowns are effectively avoided.