Abstract: In order to improve the mechanical properties and water resistance of magnesium oxychloride cement and solve the problem of resource disposal of abandoned crop highland barley straw, highland barley straw ash (HBSA) prepared by calcination and grinding under certain conditions was used to improve the mechanical properties and water resistance of magnesium oxychloride cement. First of all, the mechanical properties of magnesium oxychloride cement mortar (MOCM) with different HBSA mixing methods and amounts were tested, and the changing laws of the flexural strength, compressive strength, flexural compression ratio and softening coefficient of MOCM were tested respectively. Secondly, the pore structure and microstructure of MOCM were tested and analyzed to further explain the mechanism of the influence of HBSA on the mechanical properties of MOCM. The results show that MOCM can obtain higher mechanical properties and water resistance when HBSA is added with the external mixing method. When the content of HBSA is 5wt%, MOCM has the highest flexural strength and compressive strength; When the content of HBSA is 10wt%, the compressive strength loss of MOCM in saturated state is the smallest, and the water resistance is the best. When HBSA is added with the external mixing method and the content is 10wt%, the proportion of harmful pores and more harmful pores in the pore structure of MOCM is significantly reduced, and the proportion of harmless pores and less harmful pores is significantly increased. The hydration product Mg(OH)₂ in MOCM can react with the active SiO₂ in HBSA to generate a large number of hydrated magnesium silicate (M-S-H) gel, which effectively fills the harmful pores in MOCM, hinders the transmission and erosion of water, and improves the water resistance of MOCM.