To investigate the effect of adding highland barley straw ash (HBSA) of agricultural solid waste on the damage deterioration and microstructure of magnesium oxychloride cement (MOC) after freeze-thaw cycle erosion, MOC mortar (MOCM) modified by HBSA was prepared. By conducting macro performance tests, apparent morphology analysis, and microscopic structure testing of MOCM under freeze-thaw erosion environments with different HBSA dosages, the frost resistance evaluation parameter was used to characterize the freeze-thaw damage degradation law of MOCM modified by HBSA. Combining microscopic pore structure testing and microstructure characterization, the intrinsic reasons why HBSA affects the frost resistance of MOCM was revealed. Based on reliability theory, a one-dimensional Wiener random distribution function was used to numerically analyze the degradation process of frost resistance of HBSA modified MOCM. The research results show that after 60 cycles of freeze-thaw erosion, the degree of damage deterioration of MOCM is the lightest when the HBSA content is 10 wt%, and the frost resistance is improved by 23.48% compared to without HBSA. The micro pore structure of MOCM added with 10wt% HBSA was optimized, and the open porosity ratio decreased by 5.04% compared to without HBSA. The most probable pore size significantly decreased, and the number of internal micro pores increased. The hydration product of MOCM added with 10%wt HBSA generates hydrated magnesium silicate hydrate (M-S-H) gel, which increases the compactness of microstructure and the stability of five phase crystals in water. The one-dimensional Wiener random distribution function can be well used for analyzing the degradation process of frost resistance in HBSA modified MOCM. MOCM added with 10wt% HBSA has higher reliability in freeze-thaw damage and better frost resistance.