Highland barley straw ash contains a large amount of silica, and the ash calcined and ground under certain conditions has a higher pozzolanic effect. In order to study the effect of HBSA added into magnesium oxychloride cement mortar (MOCM) on the mechanical properties and pore structure, the activity of highland barley straw ash was studied firstly through the macroscopic mechanical properties test. Nuclear magnetic resonance (NMR) and Brunner−Emmet−Teller (BET) were used to test the distribution of full pore and micropore for MOCM respectively. The microstructure of MOCM was characterized by scanning electron microscope (SEM). The results illustrate that the highest activity of HBSA was obtained by calcining at 600℃ for 2h and grinding for 2h. The addition of HBSA has a significant effect on the mechanical properties and pore diameter distribution of MOCM. A large amount of M-S-H gel was generated in MOCM added with 10% HBSA content, and had a consequence of decreased proportion of larger pores and the increased proportion of micropores as well as the better mechanical properties and pore structure.