The invention relates to a double-layer subsurface-flow constructed wetland system used for sewage treatment in medium and small towns of cold areas and belongs to the field of sewage treatment engineering. The constructed wetland system comprises a water distribution regulating tank at one side of a tank body, an outlet water collecting tank at the other side of the tank body, an upper-layer wetland and a lower-layer wetland in the tank body, a water distribution pipe supplying water for the upper-/lower-layer wetlands, and an outlet water collecting pipe after treatment, wherein the upper-layer wetland and the lower-layer wetland are both subsurface-flow constructed wetlands and are separated by a waterproof interlayer, the lower part of the upper-layer wetland is filled with the composite filler of ceramsite and coal cinder, the upper part of the upper-layer wetland is filled with the composite filler of ceramsite, coal cinder and soil, aquatic plants are planted on the surface of the composite filler of the ceramsite, the coal cinder and the soil, the lower-layer wetland is filled with the composite filler of gravel and volcanic, the upper-layer constructed wetland is operated in summer, and the lower-layer constructed wetland is operated in winter, so that the double-layer subsurface-flow constructed wetland system is enabled to be operated stably all year round at an icebound period in northeast alpine areas, the investment and operation cost is low, the outlet water of the whole system meets the standard of first class B of GB18918-2002, and the system is particularly suitable for disperse sewage treatment in small towns with weak economic basis in cold areas.