AbstractMagnesium oxychloride cement (MOC) is a new type of cement with high early strength and good brine corrosion resistance. However, the water resistance of MOC is poor, which limits its current application. Highland barley straw ash (HBSA) contains a large amount of silica and, when calcined and ground under certain conditions, has a higher pozzolanic effect. This study investigated the effect of adding HBSA on the water resistance and pore structure of MOC mortar (MOCM), and the correlation between them. HBSA was mixed with MOCM in different proportions to test the water adsorption and water resistance. Nuclear magnetic resonance technology and the Brunauer–Emmett–Teller method were then used to test the distribution of full pores and micropores, respectively, in MOCM. Finally, the gray entropy theory was used to analyze the correlation between all the pore parameters of MOCM and the water resistance, and the numerical relationship between the softening coefficient reflecting the water resistance of MOCM and the characteristic pore parameters was determined. Results showed that the water adsorption of MOCM with added 10% HBSA decreased by 2.11%, the water resistance increased by 6.80%, and the proportion of harmful pores and more harmful pores decreased by 25.11% compared with that without HBSA. Three pore characteristic parameters—the proportion of less harmful pores, most probable diameter within the range of 20 nm, and accumulated micropore volume—had a good linear correlation with the water resistance of MOCM.