Abstract: The present study aimed to investigate the complexation mechanism between soy protein isolate fibers (SPF) and Monascus pigments (Mps), as well as the effects of different conditions on the stability of Mps within the complex. Spectroscopic methods were employed to characterize the structural properties of the complex and to evaluate the thermal, pH, and photostability of Mps within the complex. The results demonstrated that the interaction between SPF and Mps in the complex was primarily driven by hydrogen bonds and hydrophobic forces. The incorporation of Mps altered the microenvironment of SPF, reducing the hydrophobicity around tryptophan (Trp) and tyrosine (Tyr) residues and consequently modifying the secondary structure of the protein. Compared to pure SPF, in the SPF-Mps complex with a concentration of 3 mg/mL (SPM3.0), the relative content of α-helix and β-sheet in SPF decreased by 52.35% and 11.95%, respectively, while the relative content of β-turn and random coil increased by 19.04% and 69.56%, respectively. SPF enhanced the stability of Mps, as evidenced by significantly higher retention rates of Mps in the complex under various temperature, pH, and illumination conditions compared to free Mps. Specifically, under a condition of 4 ℃, the retention rates of Mps in the complex increased by 19.03% and 38.46% at absorbance wavelengths of 385 nm and 410 nm, respectively, compared to free Mps. At pH9.0, the largest increase in Mps retention rate was observed, with values of 79.21% and 230.41% at the two wavelengths. After 6 hours of illumination, the complex with a concentration of 20 mg/mL exhibited the highest increase in Mps retention rate, with values of 104.69% and 149.89% at the respective wavelengths. The findings of this study provide a theoretical basis for the color improvement and quality control of plant-based meat products.