Abstract: Slow-released plant carbon, Phragmites australis pieces, was added to the surface flow constructed wetland (SFW) to enhance the denitrification nitrogen removal of the low C/N ratio agricultural runoff. The characteristics of the simulated agricultural runoff were as follows: (8.00±1.00)mg/L N ${\mathrm{O}}_3^{-}$ -N, (9.00±1.00)mg/L TN, (0.70±0.10)mg/L N ${\mathrm{H}}_4^{+}$ -N, 0.01mg/L N ${\mathrm{O}}_2^{-}$ -N and (1.00±0.05)mg/L TP. Three pilot-scale SFWs were designed for experiment as follows: the blank SFW without Phragmites australis pieces dosage, 1 ^# SFW with 1/4 areal proportion of Phragmites australis pieces to enhanced denitrification wetland and 2 ^# SFW with 1/2 areal proportion of Phragmites australis pieces to enhanced denitrification wetland. A 40-day start-up operation was carried out in static mode. The results showed that the N ${\mathrm{O}}_3^{-}$ -N removal efficiency increased gradually to 84.2%, 89.1% and 97.8% on the 18 ^th, 18 ^th and 7 ^th day for the blank, 1 ^# and 2 ^# SFW respectively, while their TN removal efficiency gradually increased to 75.1%, 79.4% and 90.0% on the 15 ^th, 15 ^th and 7 ^th day in the early stage (1-18 d). The N ${\mathrm{O}}_3^{-}$ -N was added to (8.00 ± 1.00) mg/L when it was almost completely consumed on the 18 ^th day. In the latter stage (19-40 d), the N ${\mathrm{O}}_3^{-}$ -N removal gradually increased to 78.0%, 92.2% and 95.8% on the 40 ^th, 37 ^th and 35 ^th day for the blank, 1 ^# and 2 ^# SFW respectively, while their TN removal efficiency increased gradually to 71.4%, 75.2% and 77.1% accordingly. The start-up period of SFW could be greatly shortened by adding Phragmites australis pieces as carbon source, and 2 ^# SFW started with the higher speed. Moreover, the N ${\mathrm{O}}_3^{-}$ -N and TN removal rates of 2 ^# SFW were also higher than those of 1 ^# and the blank SFWs. It was indicated that the higher-speed start-up and better nitrogen removal efficiency could be obtained for SFW with more Phragmites australis pieces carbon dosage.