In order to understand the regulatory effect and mechanism of nitrogen application rate and density on the quality of high-quality late rice, Yuzhenxiang was used as material to conduct field experiment under four nitrogen application rates including no nitrogen(N1), 180 kg/hm2 urea(N2), 126 kg/hm2 urea(N3), 180 kg/hm2 urea+200 t/hm2 biochar(N4) and two densities including 18 cm×25 cm(D1), 14 cm×25 cm(D2), and the rice quality and leaf nitrogen metabolism characteristics were collected for further comparative analysis for each treatment. The results showed that density had no significant effect on head rice yield, amylose content, gel consistency and protein fraction, and the indexes of D2 treatments were slightly higher than those of D1 treatments. Nitrogen application increased head rice yield and protein content, reduced chalkiness grain rate and chalkiness, and N4 treatment had the best effect. The interaction effect of nitrogen application rate and density on rice quality was significant, and N4D2 treatment had the best processing quality and appearance quality, the longest gel consistency and the highest protein fraction content. Density had no significant effect on leaf nitrogen content, nitrate nitrogen content, nitrate reductase(NR) and nitrite reductase(NiR) activities, while D2 treatments had slightly higher leaf nitrogen content, NR and NiR activities. Nitrogen application increased leaf nitrogen content, nitrate nitrogen content, NR and NiR activity, and N4 treatment was the highest. The correlation analysis showed that leaf nitrogen content, NR and NiR activity were significantly or extremely significantly positively correlated with head rice rate, amylose content, gel consistency and protein fraction content, and significantly or extremely significantly negatively correlated with chalkiness grain rate and chalkiness. Increasing biochar application could improve rice quality, mainly by increasing head rice rate and decreasing chalkiness and chalky grain rate. In the studied conditions, N4D2 treatment had the best regulatory effect on the quality of high-quality rice, and its regulatory mechanism was to improve the nitrogen content in leaves and the activity of nitrogen metabolism enzymes.