In light of the observed characteristics of loose soil and low particle bonding strength in loess regions, a series of mechanical tests and indoor pot experiments were conducted. We investigated how different concentrations of casein (Cas) and sodium alginate (SA), curing periods, and moisture contents affected the mechanical properties of loess and the growth of ryegrass. Scanning electron microscopy (SEM) was used to analyze the microscopic characteristics of the soil samples prior to and following solidification. The results show that both Cas and SA can enhance the mechanical properties of loess, and the soil strength increases gradually with the increase of curing period, but the decrease of moisture content. As the proportion of Cas increases, the unconfined compressive strength (UCS) of Cas-modified loess exhibits a linear growth trend, which reaches a peak value of 352.85 kPa under Cas concentration of 5%, moisture content of 14%, and curing period of 14 days. Compared with unmodified soil, the UCS of Cas-modified loess increased by 57.92%. The UCS of SA-modified soil exhibits a parabolic trend with varying SA concentrations, reaching a peak of 353.57 kPa at 2% SA dosage. The cohesion of the Cas-modified soil increases with Cas content; whereas the SA-modified soil demonstrates more pronounced cohesion at low moisture content. The promotional effect of the two amendments with optimal concentrations on ryegrass growth are consistent with the enhancement of soil mechanical properties. Specifically, as the increase of Cas dosage, the germination rate steadily increases (reaching 96% at 2% Cas dosage), so do the stem height and root depth. At a 2% SA dosage, the promotional effects on ryegrass germination, stem height, and root depth reach their peak. According to the scanning electron microscopy (SEM) imagery, Cas strengths the soil structure by filling up the space between pores and cementation action, while the molecular chains of SA can form an “egg carton”-shaped three-dimensional gel network to enhance the overall stability. The findings of this study can serve as a reference for the design and implementation of slope improvement projects in loess regions.