This study aimed to explore the release mechanism of bound polyphenols (BP) from the insoluble dietary fiber (IDF) in carrots via mixed solid-state fermentation (MSF) using Trichoderma viride and Aspergillus niger. The results indicated that BP released by MSF (80.8759 mg GAE per 10 g DW) was significantly higher than that by alkaline hydrolysis. In addition, 17 polyphenols were detected and their biotransformation pathways were proposed. Quantitative analysis showed that MSF released numerous p-coumaric and organic acids, which led to both an enhancement in α-amylase inhibitory activity and elevated antioxidant enzyme activity in Caenorhabditis elegans (C. elegans). Furthermore, the dynamic changes in the carbohydrate-hydrolyzing enzymes and the structural characteristics indicated that the destruction of hemicellulose, the deposition of lignin and the secretion of xylanase were vital for the release of BP. Overall, this study demonstrated that MSF is beneficial for the release of BP from IDF, which could provide new insight into the utilization of agricultural byproducts in a more natural and economical way.