Riboflavin sodium phosphate has been confirmed as a promising biomass product derived from natural plants. In this paper, a novel method of dyeing and multifunctional modification of silk fabric by impregnation with riboflavin sodium phosphate was proposed, such that protein silk fabric can be endowed with bright yellow color and multi-functionality. The results of this paper confirmed that the pH and concentration of riboflavin sodium phosphate solution are critical factors for dyeing and multifunctional modification. Attractively, the photochromic performance was one of the most distinctive features of the modified silk fabric, and the dyed silk fabric turned into fluorescent green from original yellow under 365 nm ultraviolet lamp. Furthermore, the modified silk fabric exhibited good antibacterial properties with a high inhibition rate of 92% for Escherichia coli. Besides, the flame retardancy of silk fabric was significantly improved after modification. The damaged length of modified silk fabric with 40% owf riboflavin sodium phosphate was lower than 10.4 cm and passed the B₁ classification. As revealed by the result of this paper, riboflavin sodium phosphate is sufficiently effective in serving as an eco-friendly multifunctional agent for strengthening the add-value of silk textiles.