In order to reveal the nitrogen uptake and utilization of rice roots under water and biochar management, field plot experiment and 15N tracer micro plot were used. Two irrigation modes (dry-shallow-wet irrigation and conventional flooding irrigation) and four straw biochar application levels (0t/hm2, 2.5t/hm2, 12.5t/hm2 and 25t/hm2) were set up. The effects of straw biochar on the morphological and physiological characteristics of rice roots, and the absorption and utilization of fertilizer and soil nitrogen by roots under dry-shallow-wet irrigation were studied. The results showed that the application of straw biochar changed the morphological and physiological characteristics of rice roots, and the appropriate amount of straw biochar increased the main root length, root volume, root fresh weight, root active absorption area, root bleeding velocity and root activity, optimized the root-shoot ratio, which was conducive to nitrogen absorption; the uptake of fertilizer-15N and soil nitrogen by rice roots under dry-shallow-wet irrigation mode was also improved. There was a significant positive correlation with root bleeding and root activity (P < 0.01), a significant positive correlation with active absorption area (P < 0.05), and a significant negative correlation with root-shoot ratio (P<0.05). The changes of root morphological and physiological characteristics in dry-shallow-wet irrigation mode promoted the uptake of fertilizer-15N and soil nitrogen by rice roots, and improved rice yield and nitrogen use efficiency. Compared with the treatment without straw biochar, the economic yield, nitrogen absorption and utilization efficiency, agronomic utilization efficiency and partial productivity of nitrogen fertilizer in the treatment of dry-shallow-wet irrigation with 12.5t/hm2 straw biochar were increased by 13.05%, 30.54%, 11.67% and 13.05%, respectively. The results can provide theoretical basis and technical support for the application of straw biochar in paddy field in cold and black soil region.