To investigate the mechanism of influence of low air pressure curing on the micropore structure and impermeability of cement mortar， the low air pressure environment chamber was used in curing cement mortar， and the mechanical properties， impermeability， pore structure， micromorphology and hydration products of mortar under low air pressure （0.2 P₀， 0.5 P₀， 0.8 P₀， 1.0 P₀， where P₀ is the standard atmospheric pressure） were tested and analyzed. The results show that low curing air pressure degrades the impermeability and mechanical properties， and the mechanical properties decline most seriously in the range of （0.5-0.8） P₀. Air content， pore spacing coefficient and the proportion of large pores of cement mortar cured in low air pressure increases， and the pore structure deteriorates significantly. Micropore structure of hardened cement mortar has a significant effect on its impermeability. With the decrease of curing air pressure， the air content， pore spacing coefficient and average pore aperture increase， resulting in the attenuation of impermeability. The cause of it includes the gel structure， water evaporation， hydration products and the introduction of bubbles.