Spring frost involves low temperatures in spring. Research shows cold snaps can alter herbaceous plants' biomass allocation, impacting grassland ecosystems. However, the exact effects of frost timing and severity remain unclear. This study simulated spring frost based on characteristics of spring frost on the northern slope of the Tianshan Mountains to examine how alfalfa (Medicago sativa) and ryegrass (Lolium spp.) adjust biomass allocation under varying frost intensities and timings, including interspecific differences in these responses. The findings revealed that (1) compared to the control group (which did not undergo low-temperature treatment), alfalfa was more sensitive to high-intensity spring frost, exhibiting a significant decrease of 13.6% in the root weight ratio and increases of 8.65% and 4.96% in the stem and leaf weight ratios, respectively. In contrast, ryegrass displayed an inverse trend, although the changes were not significant. (2) Early stage spring frost (immediately after thinning) significantly affected alfalfa biomass allocation, leading to an 11.28% decrease in the root weight ratio, whereas it also significant increases of 3.78% the stem weight ratio by 7.51% and leaf weight ratio. In contrast, late stage spring frost (applied on the 17rd day after thinning) had a relatively greater effect on ryegrass, with the root weight ratio increasing by 4.13% and the stem weight ratio decreasing by 4.18%. These findings reveal plants' distinct adaptations to cryogenic stresses, improving our understanding of herbaceous growth responses to extreme weather in arid zones and offering data to support grassland ecosystem services in Xinjiang.