Abstract: The effect of different nitrogen fertilizer levels on the yield and quality of Brassica oleracea and NO₃^--N content of soil in Beijing was studied through field experiment. The results showed that: (1) the content of NO₃^--N in soil increased with the increase of nitrogen fertilizer level in 3 sampling periods in 0-60 cm soil, the content of NO₃^--N in 0-30 cm soil showed a decline tendency with the growth of the crops, while the content of NO₃^--N in 30-60 cm soil showed an increase tendency, and the content of NO₃^--N in 60-90 cm soil slightly changed; (2) the yield of Brassica oleracea showed an increase-decrease trend with the increase of nitrogen inputs, and the yield was the highest when the application amount of nitrogen was 240 kg/hm²; (3) the absorption amount of nitrogen by Brassica oleracea showed an increase-decrease trend with the increase of the nitrogen fertilizer application amount, the absorption amount of nitrogen of Brassica oleracea under nitrogen fertilizer treatments were significantly higher than that under the treatments without nitrogen fertilizer, and the NO3-- N content in leaves were also higher and different among treatments with different nitrogen fertilizer application levels. In conclusion, the addition of nitrogen fertilizer had a great effect on the content of NO₃^--N in 0-60 cm soil, and the suitable amount of nitrogen application (240 kg/hm²) could effectively improve the yield of Brassica oleracea and its absorption amount of nitrogen.