In this study, we used ‘Ningjing No. 6’ as the material, treated with exogenous fulvic acid (FA) and auxin inhibitors to explore the effect of FA on root growth and its relationship with auxin. The results showed that: (1) after treatment of rice seedlings with 50~800 mg·L^-1 FA for 6 days, when the concentration of FA exceeded 100 mg·L^-1, the root elongation was significantly promoted; when the concentration of FA exceeded 400 mg·L^-1, the average lateral root length and lateral root density of rice in FA treatment group increased significantly compared with the control. (2) Compared with the control, lowconcentration FA treatment had no significant effect on the content of auxin in the root tips of rice seedlings, but 400 mg·L^-1 FA treatment significantly increased the content of endogenous auxin in the root tip of rice seedlings. (3) 3 μmol·L^-1 auxin synthesis inhibitor 4biphenylboronic acid (BBo), 4phenoxyphenylboronic acid (PPBo) or 30 μmol·L^-1 auxin signal transduction inhibitor pchlorophenoxy isobutyric acid (PCIB) treatment significantly inhibited the growth of rice roots and the occurrence of lateral roots; 1 μmol·L^-1 auxin polar transport inhibitor triodobenzoic acid (TIBA) treatment significantly inhibited the elongation and growth of rice roots and the occurrence of lateral roots, but has no significant effect on the lateral root length. (4) The cotreatment of FA with BBo or PPBo could significantly inhibit the promotion of FA on rice root elongation and lateral root formation. TIBA and PCIB treated rice together with FA could inhibit the promotion of FA on the growth of roots. In addition, PCIB could repress the promotion of FA on lateral root development, but TIBA has no significant effect. These results indicate that exogenous FA may regulate root elongation and the development of lateral roots by regulating the synthesis, polar transport or signal transduction of endogenous auxin.