Silk fibroin is an excellent raw material for medical products as it shows remarkable biocompatibility, water-based processing, and biodegradability. Cordyceps militaris (C. militaris) polysaccharide (CMP) shows strong antioxidant and anti-UV activities. To improve the bioactivity of silk fibroin, we developed a complex of silk fibroin oligopeptide (SFO) with CMP and characterized the mechanism of tyrosinase inhibition. The optimal composition was determined to be 1.0 mg/mL SFO and 0.6 mg/mL CMP. At these concentrations, the inhibition rate of tyrosinase was 95.09%, the chelating ability of ferrous ions was 93.68%, the water evaporation rate was 17.08%, and the UV transmittance was 1.69%. The tyrosinase inhibition mechanism of the complex was studied in mouse B16 melanoma cells. The results showed that the inhibition rates for tyrosinase and melanin were 63.25% and 46.86%, respectively. Melanin synthesis-related genes MITF, TYR, and TRP-1 were downregulated by 42.17%, 88.97%, and 63.35%, respectively. Overall, our results suggest that the complex of SFO and CMP inhibits melanin synthesis by inhibiting tyrosinase activity and down regulating melanin related genes and can be used to produce healthy medical products.