For understanding the effect of ferrous iron addition on contaminant removal in different layers of horizontal subsurface flow constructed wetlands, the nitrogen and COD removal and their mechanisms were investigated in this study. The results showed that addition of ferrous iron was beneficial to the removal of total nitrogen, nitrate nitrogen and ammonium nitrogen, and the removal of nitrogen was optimized at the ferrous iron concentration of 50 mg/L. However, the excessive ferrous iron addition inhibited nitrogen removal, especially for the removal of ammonium nitrogen in the lower layers, while the nitrogen removal with ferrous iron addition was greater than that without ferrousiron. As well, ferrous iron addition altered the layering effect of the three inorganic forms of nitrogen. At the beginning of the experiment, ferrous iron addition improved the COD removal, affecting the COD removal in the different layers. Iron concentrations in the different layers dropped rapidly after the input of ferrous iron, while pH values in the effluent increased first and then kept in a stable trend due to the valence structure and hydrolytic characteristics of iron. The pH value in the lower layer was lower than those in the upper and middle layers. Iron properties also caused the changes of oxidation reduction potential (ORP) in different layers. Addition of ferrous iron promoted the layering effect of ORP, but slowed down the decrease of ORP.