Abstract: In order to explore the effect of highland barley straw ash (HBSA) on the durability and pore structure of magnesium oxychloride cement (MOC), HBSA was used to improve the durability of MOC, and highland barley straw ash–magnesium oxychloride cement composites were prepared. The durability of magnesium oxychloride cement mortar (MOCM) with different HBSA contents were studied under the conditions of salt lake brine erosion, freeze-thaw cycle erosion and salt–frozen coupling erosion. Three durability evaluation indexes: Relative mass evaluation parameters, relative dynamic elastic modulus evaluation parameters and relative compressive strength evaluation parameters were used to reflect the durability deterioration law of MOCM, and determine the optimal content of HBSA. Through the analysis of apparent morphology and pore structure test, the durability damage degree and pore structure characteristics of MOCM under different erosion conditions were revealed. The results show that the durability damage of MOCM caused by freeze-thaw cycle erosion is more serious than salt brine erosion and salt-frozen coupling erosion, and more macro cracks are produced on the surface of MOCM specimens. The addition of HBSA can significantly improve the durability of MOCM. When the content of HBSA is 10wt%, the durability of MOCM under salt lake brine erosion, freeze-thaw cycle erosion and salt–frozen coupling erosion is 21.24%, 23.48% and 18.91% higher than that without HBSA, respectively. The opening porosity of MOCM added with 10wt%HBSA decreases, the specific surface area increases, and the most probable pore diameter and average pore diameter decrease, which refines the pore structure of MOCM and improves the durability.