The effects of three ultrafiltration-recovered soy protein hydrolysate fractions (USHFs) with different molecular weights (>10 kDa for USHF-1, 1-10 kDa for USHF-2, and <1 kDa for USHF-3) on mulberry anthocyanin extract (MAE) was investigated under the condition of neutral pH for the first time in this study. Generally, all USHFs showed pronounced color- and anthocyanin-stabilized effects. The addition of 1 mg/mL of USHF-3 showed best protection for anthocyanin stability in this study. Based on the spectroscopic results, the electrostatic interactions were the main forces in the binding system between USHF-3 and cyanidin-3-O-glucoside (C3G) indicated by the negative ΔH and positive ΔS values. USHF-1 and USHF-2 and their complexes with C3G were mainly driven by hydrophobic interactions determined with positive ΔH and positive ΔS values. C3G addition resulted in tertiary structural changes of the USHFs. Overall, the complexation of USHFs and C3G participates in maintaining anthocyanin stability.