To study the feasibility of using magnesium oxychloride cement mortar （MOCM） modified by highland barley straw ash （HBSA） as an outer protective layer of ordinary concrete in the western salt lake area to resist brine erosion in the salt lake area and prolong the service life of the concrete， experimental research on the bonding strength of MOCM mixed with HBSA under water immersion environment was carried out. Ordinary concrete was used as the bonding base， and the influence factors such as the application of interface agent on the concrete base， the mixing of HBSA in MOCM， and the thickness of the mortar layer were used as variables. The bond pull-out tests were carried out to analyze the effect of each factor on MOCM bonding strength and to determine the optimal design parameters. Through numerical simulation methods such as polynomial simulation and grid processing in MATLAB， a time-varying model of magnesium oxychloride cement bond strength was established， and the damage degradation law of the bond strength of modified MOCM under water immersion environment was further analyzed. The phase composition， functional group structure， microscopic morphology， element mapping， and other characteristics of MOCM were analyzed by microscopic testing technology， and the mechanism of the influence of HBSA on the bonding performance of MOCM was revealed. The results show that there is more active SiO2 in HBSA， which can undergo a secondary hydration reaction with the hydration products in MOCM to form hydrated magnesium silicate （M―S―H） gel， which can fill the internal pores of MOCM， enhance the compactness， and improve the adhesive properties. The MOCM with a thickness of 18 mm， mixed with HBSA， and coated with interface agent has the highest bond strength， the bond strength degradation rate is the slowest in water immersion environment， and the numerical model based on the cubic polynomial can better reflect its bond strength. The degradation law of the optimal group YY-18 has a correlation coefficient R2 of 0.98.