Due to alkali-activated slag-red mud cementitious material (AASR) with high water absorption rate, water and erosive ions are easily enter into AASR interior, which poses a great threat to AASR durability. Sodium stearate (NaSt) was used to improve the pore structure of AASR, and a water-repellent film was introduced on the pore wall, which reduces its water absorption rate of AASR. The improvement mechanism of NaSt on the water absorption rate of AASR was revealed by analysing of the pore structure, contact angle and micromorphology of AASR. The results shows that NaSt introduces more closed pores into AASR interior and the connectivity of the internal pores will be blocked by these closed pores, thus the water transport is blocked in the AASR pore size. A water-repellent film formed by NaSt is formed in the capillary wall, which increases the contact angle of the pore wall and reduces the capillary suction. The reduction of initial water absorption rate of AASR with adding 0.5% (mass fraction) NaSt is up to 83%, but it has less effect on mechanical properties.