In this paper, compressive and flexural strength tests of seawater sea sand mortar (SSM) with different curing ages and water-cement ratios were carried out. The influences of water-cement ratio, curing age and water temperature on capillary water absorption properties of SSM were studied by weighing method. The relationship between mechanical properties or capillary water absorption properties and corresponding microstructure of SSM was studied based on low field nuclear magnetic resonance technology. Finally, the representative capillary diameter calculated based on the water absorption model of porous media capillary was compared with the pore diameter estimated based on the transverse relaxation time of low field nuclear magnetic resonance technology. The results show that the strength of SSM developed rapidly in the first 3 days, and the compressive and flexural strength of SSM specimens with water-cement ratio of 0.4 curing for 3 d are 56.2% and 70.3% of those of SSM specimens curing for 28 d. When the water temperature rises from 20 ℃ to 40 ℃, the capillary water absorption coefficient of SSM specimen with water-cement ratio of 0.4 curing for 28 d increases 1.2 times. In the long-term one-dimensional water absorption process, the cumulative capillary water absorption volume per unit area of specimen is linearly related to the 0.25 power of the water absorption time. The porosity of SSM specimens decreases with the increase of curing age. The typical capillary diameter based on capillary water absorption model is close to the mortar pore diameter estimated based on transverse relaxation time of low field nuclear magnetic resonance.