The invention discloses a combined constructed wetland system. The combined constructed wetland system successively comprises a water adjusting and distribution tank, vertical flow constructed wetlands, a horizontal flow constructed wetland and a surface flow constructed wetland, wherein two groups of vertical flow constructed wetlands are provided and are alternatively operated. The invention also discloses a method for treating the low-concentration heavy metal sewage by utilizing the combined constructed wetland system. By adopting the combined constructed wetland system, the conventional pollutants and heavy metals can be synergistically removed, the treatment effect is stable and efficient, the removal rate of COD, ammonium nitrogen, total nitrogen, total phosphorus, hexavalent chromium and total nickel can respectively reach 60 percent, 80 percent, 70 percent, 65 percent, 50 percent and 50 percent or more, CODCr is less than or equal to 20 mg / L, NH3-N is less than or equal to 1.0mg / L, TN is less than or equal to 1.0 mg / L, TP is less than or equal to 0.2 mg / L, the hexavalent chromium is less than or equal to 0.05 mg / L, and the total nickel is less than or equal to 0.02 mg / L.The operation cost is less than 0.2 Yuan per ton. The combined constructed wetland system is flexible in operation, convenient in management, free from secondary pollution and significant in environmental benefit and social benefit.