Gray mold caused by Botrytis cinerea is a major postharvest disease of table grapes that leads to enormous economic losses during storage and transportation. The objective of this study was to evaluate the effectiveness of fulvic acid on controlling gray mold of table grapes and explore its mechanism of action. The results showed that fulvic acid application significantly reduced downy blight severity in table grapes without exhibiting any antifungal activity in vitro. Fulvic acid induced phenylpropanoid metabolism, as evidenced by accumulation of phenolic compounds and flavonoids, higher activities of phenylalanine ammonia-lyase (PAL), cinnamate-4-hydroxylase (C4H) and 4-coumarate-CoA ligase (4CL), up-regulation of genes related to phenylpropanoid biosynthesis (PAL, C4H, 4CL, STS, ROMT and CHS). Our results suggested that fulvic acid induces resistance to B. cinerea mainly through the activation of phenylpropanoid pathway and can be used as a new activator of plant defense responses to control postharvest gray mold in table grapes.