The study aids to clarify the effects of biochar on root growth and soil microbial population of flue-cured tobacco under the condition of nitrogen reduction.The field test method was used to study the addition of different amounts of biochar under a 40% nitrogen reduction method，and the root morphology and vigor，soil microbial population of flue-cured tobacco were analyzed.The results showed that the application of biochar effectively increased the root morphological indexes of tobacco plants，and at the same time enhanced the root activity，and significantly promoted the increase of soil enzymes and bacteria.Compared with the control group，under root length T2(1200 kg/hm²)increased by 31.12%，root tip number increased by 31.66%，root surface area increased by 55.98%，root volume increased by 35.14%；root activity increased by 78.57%；soil mold and bacteria number increased by 34.29% and 34.17%，separately.Therefore，biochar materials can promote the development of tobacco plant root system morphological indicators under 40% nitrogen reduction，increase root vitality，and increase the number of soil microorganisms，which is conducive to the sustainable development of green tobacco leaves.