The invention relates to a combined subsurface-flow constructed wetland system with high efficiency and low cost for processing polluted river water. The system is formed by combing a first-stage vertical subsurface-flow wetland and a second-stage horizontal subsurface-flow wetland, wherein the vertical subsurface-flow wetland comprises a box body (1), a water inlet distribution pipe (2), vertical stand pipes (3), a water outlet collection pipe (4), a water outlet hose (5), a heightened base (6), a furnace slag packing layer (7) and a gravel packing layer (8); the horizontal subsurface-flow wetland comprises a box body (9), a water inlet distribution pipe (10), a porous barrier (11) of a water inlet distribution area, a porous barrier (12) of a water outlet area, a water outlet collection pipe (13), a water outlet hose (14), fixed water level outlets (15), a furnace slag packing layer (16) and a gravel packing layer (17). By adopting the combined subsurface-flow constructed wetland system, total nitrogen and phosphorus in the river water can be effectively removed, the height of the water outlet hose of the vertical subsurface-flow wetland can be adjusted to change the water level of the wetland and the dissolved oxygen content, and the nitrification effect or the denitrification effect can be strengthened; and the cost of furnace slag packing is low.