Plant tannins, polyphenolic plant secondary metabolites are involved in important chemical defense processes in plants. In this study, tannic acid was used as the standard of plant tannins to determine the effects on nutritional indices and activities of glutathione S-transferases (GSTs), cytochrome P450 monooxygenase (CYP450), carboxylesterase (CarE), and acetylcholinesterase (AChE) in fourth-instar larvae of Hyphantria cunea (Drury) by feeding on an artificial diet containing tannic acid under different treatments. We found that tannic acid significantly affected the digestive capacity and food utilization rate of H. cunea larvae. A tannic acid concentration of less than 2.0% promoted feeding and the utilization of undesirable food by H. cunea larvae, while inhibitory effects were observed at high concentrations (>2.5%). Tannic acid had a significant effect on the activity of detoxification enzymes and AChE in H. cunea larvae in concentration-dependent and time-dependent manners (P < 0.05). These results provide new insights into the potential mechanisms underlying detoxification in H. cunea larvae against tannic acid in host plants.