Abstract: To reveal the mechanism of tobacco stalk biochar on root growth of flue-cured tobacco, field experiments were carried out at three biochar application rates (0, 1, 10 and 50 t/ha) and the root growth of flue-cured tobacco and soil physicochemical properties were measured. The results showed that biochar promoted the root growth of flue-cured tobacco, both the volume and dry weight of roots increased at an early stage. Via increasing biochar application rate, root bleeding intensity of flue-cured tobacco increased, however, the active absorption area (AAA), percentage of AAA and dry weight of root increased first and then decreased. The optimal morphological and physiological characteristics of roots and the highest yield of flue-cured tobacco were obtained at the biochar application rate of 10 t/ha. With the increase of biochar application rate, the mass water content, capillary capacity, gas phase ratio and available phosphorus and potassium contents in soil increased. However, the alkaline nitrogen content and catalase activity increased first and then decreased. Comparing with the control, the available potassium content in soil increased by 11.0%-382.3% under the biochar treatment.