In this study, the innovative use of sericin peptide (SRP) as an anti-hardening ingredient was studied and investigated for the anti-hardening effect on high protein nutrition bars (HPNBs) made from soybean protein isolate (SPI). Texture analysis showed that the addition of SRP to the SPI-HPNBs alleviated the hardening of the samples during early storage (72 h). The mobility of water and small molecules in the samples were improved by the addition of SRP. CLSM, SEM and AFM showed that SRP slowed down the phase separation in the samples and maintained the microstructure. The addition of SRP resulted in changes in ζ-potential, protein secondary structure content and surface hydrophobicity, which prevented self-aggregation of proteins. Overall, SRP could effectively hinder the phase separation of SPI-HPNBs caused by water migration and protein self-aggregation during early storage and alleviate the hardening, and this effect increased with the increase of SRP concentration. These results suggest that SRP can be used as a promising anti-hardening ingredient in the food industry to improve the texture of food products.