Application of biochar in the loess arid area has important theoretical value for improving the rhizosphere nutrient environment, optimizing the research on regulation mechanism of millet root construction and biomass allocation, in this study, Jingu 21 was taken as the research object, under potted simulation conditions, five treatments including 3 g/kg of nitrogen fertilizer (3N, control), 5 g/kg of nitrogen fertilizer(5N), 10 g/kg of nitrogen fertilizer(10N), 5 g/kg of nitrogen fertilizer and 10 g/kg of biochar(5N10M), and 10 g/kg of nitrogen fertilizer and 20 g/kg of biochar(10N20M) were set. The effects of different biochar combined with nitrogen fertilizers on the root construction, the physiological function in the leaves, and biomass accumulation of millet were comprehensively analyzed. The results showed that compared with the control, the root length and total root surface area of millet treated with 5N10M were significantly increased by 94.17% and 71.25%, respectively, while the root length and total root surface area of 10N20M were significantly decreased by 38.72% and 31.01%, respectively, compared with 5N10M. Appropriate combination of biochar and nitrogen fertilizers had regulation mechanism of the morphological development of millet roots, and there was an optimal ratio threshold. Compared with CK, the photosynthetic rate of leaves at jointing stage, booting stage, heading stage, and filling stage of 5N10M treatment increased by 95.40%, 85.25%, 79.23%, and 61.97% respectively, and the photosynthetic function of leaves was significantly improved, and the water use efficiency of leaves was significantly increased by 2.23%-13.98%. Combined application of biochar and nitrogen fertilizer could effectively regulate the accumulation and allocation of biomass in roots, stems, and leaves of millet. On the heading stage and filling stage, the dry weight of plant under 5N10M treatment was significantly increased by 11.86% and 24.15% compared with that under 5N treatment, and the dry weight of plant under 10N20M treatment was increased by 9.43% and 23.43% compared with that under 10N treatment. To sum up, reasonable combined application of biochar and nitrogen fertilizer could significantly increase the root surface area and root length of millet, promote the absorption of nitrogen by leaves, improve the level of leaf development and function, and the chlorophyll content of combined application of biochar and nitrogen fertilizer was more stable, which was beneficial to the formation of millet photosynthetic products and the optimal allocation of biomass. 5 g/kg of nitrogen fertilizer and 10 g/kg of biochar had a good regulatory effect on the accumulation of plant biomass. [ABSTRACT FROM AUTHOR]