The invention discloses a double-layer ecosystem for wastewater treatment in cold and arid areas. A main body comprises a vegetation restoration layer above a frozen soil layer, an underflow layer anda seepage-proofing layer, wherein the underflow layer and the seepage-proofing layer are arranged below the frozen soil layer; the underflow layer comprises a front end gabion part and a multi-mediumpacking layer, a water distribution well and a water receiving well are arranged on the two sides of the underflow layer, and the water distribution well is provided with an aeration system and a single-sided water distribution tracery wall and a filtering wall; a water outlet collecting pipe is composed of a water outlet collecting branch pipe with holes and a water receiving header pipe; and anemptying valve is arranged at the bottom of the water receiving header pipe, and the water receiving well is provided with an internal circulating pump. Water after treatment in the water distribution well is adopted by the upper-layer vegetation restoration layer to conserve salt-resistant, fluorine-resistant and cold-resistant plants, the soil quality is improved, and vegetation is restored; agap is reserved between an upper layer and a lower layer, so that permeated water after the upper-layer plants are irrigated enters a lower-layer underflow wetland, the water is further purified, andthe water loss is avoided; the lower-layer wetland is located below the frozen soil layer where the ecosystem is located, after waste water passes through a front end gabion layer to be purified in avertical flow mode under the effect of gravity firstly, a water flow goes forward in a self-driving mode along an infiltration curve in a horizontal underflow mode at the middle section and finally enters the water receiving well at the bottom of a bed layer through a water receiving pipe, the operation temperature of the underflow layer is kept to be 8-15 DEG C in winter, and the freezing problemin the cold areas is solved.