Treatment performance of subsurface constructed wetland treating polluted river water in winter was studied. When hydraulic loading fell down from 30cm/d to 15cm/d, the average removal rates of ammonium nitrogen and COD rose up from 14% and 20% to 39% and 31% respectively. Water temperature had great influence on ammonium nitrogen removal. Membrane overlay on the wetland could improve pollutants removal efficiently. The average removal rates of ammonium nitrogen and COD could rise up from 29.4% and 29% to 67.6% and 46.6% respectively. Microorganism enzyme activity experiments show that increase of microorganism activity caused by membrane overlay contributed a lot to pollutants removal enhancement. Urease activity rose up from 0.025 mg/(g x d) to 0.037 mg/(g x d) with membrane overlay, and dehydrogenase activity rose up from 0.17 microL/(d x g) to 4.54 microL/(d x g) with membrane overlay. The pollutants releasing experiment of aquatic plants show that faded plants would release a lot of pollutants into water. Therefore, plants harvest was necessary in autumn.