In this paper, the effects of sea water and mineral admixture on the mechanical properties and apparent porosity of sulphoaluminate cement (SAC) mortar, the properties such as setting time, chemical shrinkage, pH value of pore solution and chloride ion binding capacity of paste were studied. Furthermore, the hydration products and the microstructure of SAC were analyzed by XRD, SEM and EDS. The results show that sea water accelerates the early hydration of SAC and improves its early strength, while there is no significant difference in the long-term strength of cement mortar between sea water and fresh water. Both fly ash and silica fume prolong the setting time of SAC, which is disadvantageous to the early compressive strength. 28 d compressive strength of SAC mortar improves with the silica fume mass fraction of 5.0% and 7.5%. As the silica fume content increases, the water consumption, chemical shrinkage and apparent porosity of SAC increase. However, the fluidity, the pH value and the chloride ion binding amount of cement paste reduce with the increase of silica fume content. Meanwhile, Fly ash improves the fluidity of mortar and reduces the chemical shrinkage of cement paste. But the adverse effect on the compressive strength and flexural strength of SAC mortar is more obvious with the increase of fly ash content. When 10% (mass fraction) fly ash is added, the chloride ion binding amount increases and the apparent porosity reduces slightly.