Abstract: The influence of mold living activity on the basic structure and plant stimulation of fulvic acid was studied. It was shown that after mold treatment, the stimulation of fulvic acid was weakened. By comparing the IR spectrum, it was found that mold performed a key function in the transformation from fulvic acid to humic acid. Besides normal chemical action, the biochemical activity of microbiology performed an important function during the aging from fulvic acid to humic acid. Fulvic acid can be used as nutrition sources for mold living. In the course of living action, a series of changing takes place in fulvic acid chemical constitution and structure. The content of carboxyl group decreases and hydroxyl group increases. The quinone group and carbonyl group have no evident change and the E4/E6 value decreases. The results show that, because of the mold living activity, condensation polymerization takes place between fulvic acid which makes fulvic acid change into humic acid. After mold treatment, the IR spectrum of fulvic acid tends to change to that of its congenetic humic acid. Mold consumes the physiological active materials of fulvic acid during its living activity and changes its molecular structure. As a result, mold decreases its physiological activity, which might have a relationship with the carboxyl group in fulvic acid molecules.