The invention provides a composite-electrode horizontal subsurface constructed wetland device for the efficient nitrogen removal of low C/N domestic sewage, which is characterized by comprising a multi-medium filter bed, at least two activated carbon fiber electrode assemblies are distributed in the multi-medium filter bed, the activated carbon fiber electrode assemblies are connected with a direct-current supply system, the two sides of the multi-medium filter bed are respectively provided with an inflow water distributor and an outflow water distributor, an emergent aquatic plant set is planted in the multi-medium filter bed, and the inflow water distributor and the outflow water distributor are connected with a water supply system. According to the invention, a micro electric field can be formed so as to promote the plant growth and the radial oxygen loss, thereby facilitating the formation of an aerobic nitrification environment. Meanwhile, an activated carbon fiber column can form a heterotrophic-autotrophic combined denitrifying bacterium biofilm, thereby reducing the demands on an organic carbon source, and improving the rate of nitrogen removal by denitrification. The horizontal subsurface constructed wetland device disclosed by the invention is simple in structure, a carbon source is not required to be added, environments facilitating nitrification and denitrification can be formed simultaneously, the nitrogen removal rate is high, the energy consumption is low, the device is uneasy to be blocked, and the maintenance and replacement of the activated carbon fiber electrode assemblies are convenient.