A pilot plant of subsurface wastewater infiltration system (SWIS) filled with red clay was tested to treat rural sewage with hydraulic loading of 2 cm/d. The experiment results showed that average removal rates of COD, NH4(+)-N, total phosphorus, and total nitrogen were 84.7%, 70.0%, 98.0% and 77.7%, with average effluent concentrations of COD, NH4(+)-N, total phosphorus, and total nitrogen being 11.7 mg/L, 4.0 mg/L, 0.04 mg/L and 4.7 mg/L respectively, which met the standard for water reuse issued by the Ministry of Construction of China. Nitrogen removal mechanism analysis revealed that nitrogen in the influent was mainly eliminated through biological removal via nitrification/denitrification processes. In the pilot system, denitrification was well proceeded but nitrification was not satisfactory. How to improve nitrification performance through change of soil environment was the key to enhance nitrogen removal rate. The measurement results of redox potential in the filled soil showed that reductive property of soil was main hindrance to nitrification process.