The slag composite cementitious material was used to solidify the coastal saline soil. Through the unconfined compressive strength test, water stability test, soluble ions content test, SEM analysis, EDS energy analysis and XRD analysis, the unconfined compressive strength, water stability and soluble ions content of solidified soil were tested, the microstructure of solidified soil was observed, the hydration products of solidified soil was analyzed, the solidification mechanism of saline soil solidified by the slag composite cementitious material was clarified. The results show that the 28d and 90d unconfined compressive strength of solidified soil are relatively high, the water stability of solidified soil is relatively good, and can meet the requirements for the general engineering foundation solidification processing. The soluble Cl- and SO2-4 in saline soil take part in the hydration reaction of slag composite cementitious material, and the crystals of 3CaO·Al2O3·(05CaCl2·05CaSO4）·12H2O in the shape of lamella and needle are generated, thereby the soluble Cl- and SO2-4 content in the solidified soil are reduced, the unconfined compressive strength and water stability of solidified soil are improved.