Spodoptera exigua has emerged as a globally agricultural pest due to its strong adaptability to diverse nutritional conditions. In this study, we used artificial diets with a protein-to-carbohydrate (P:C) ratio of 1:1 as the control and examined the effects of extreme nutritional imbalances (P:C = 1:7 or 7:1) on survival and the activities of nine enzymes in third-stage larvae. The results showed that survival rates of larvae in both unbalanced diet groups were lower than those in the control group. High-carbohydrate-low-protein diets enhanced carboxylesterase, polyphenol oxidase, superoxide dismutase, peroxidase and three digestive enzymes (trypsin, amylase, lipase), cope with overeating and peroxide accumulation. The high-protein-low-carbohydrate diet exclusively increased lipase activity, confirming that larvae compensate for carbohydrate deficiency through lipid mobilization. These findings provide novel insights into polyphagy mechanisms in S. exigua, establishing a theoretical basis for predicting pest dispersal and developing control strategies.