In order to explore the effects of biological nitrification inhibitor syringic acid on the root at seedling stage growth of tobacco varieties K326 and Yunyan 85, a substrate culture experiment was conducted with six concentrations of syringic acid (0, 10, 25, 50, 100 and 200 μmol/L) to investigate the effects of different syringic acid concentrations on the elongation of the primary roots and lateral roots of K326 and Yunyan 85 at different time (3 d and 5 d). The results showed that compared with the control (0 μmol/L), syringic acid in the range of 25-100 μmol/L promoted the primary root elongation of K326 at 3 d and 5 d by 13.33%-30.67% and 8.54%-22.55%, respectively, with the optimum concentration of 50 μmol/L, while syringic acid in a range of 10-50 μmol/L promoted the primary root length of Yunyan 85 at 3 d and 5 d by 7.81%-18.75% and 4.10%-10.66%, respectively, with the optimum concentration of 25 μmol/L. The promotion of syringic acid on the primary root elongation of K326 and Yunyan 85 was better at 3 d than at 5 d. Low concentration of syringic acid significantly increased the numbers of lateral roots of K326 and Yunyan 85, with the optimum concentration of 25 μmol/L for both varieties. Correlation analysis showed that the change rates of main root elongation and lateral root numbers of the two tobacco varieties were significantly positively correlated with syringic acid. It can be seen that the appropriate syringic acid concentration can promote the growth of main roots and lateral roots of two tobacco varieties at seedling stage. Biological nitrification inhibitor syringic acid has the potential to promote the growth of tobacco roots, providing a theoretical basis for the development of new fertilizer for flue-cured tobacco.