Cement and fine slag composite stabilizer were used to stabilize the coastal saline soil. The unconfined compressive strengths of 7, 28, 90d of stabilized soil were tested, and the stabilization mechanism was investigated in terms of the loss on ignition, the content of hardened materials, and the means of XRD,TG DSC and SEM. The results shows that the stabilizing effect of cement and fine slag composite stabilizer is better than that of simply stabilizer, and when the content of fine slag is 0%75%, with the increase of the amount of slag in the stabilizer, the 7，28d unconfined compressive strength of stabilized soil is gradually improved, which is up to 6.0，10.0MPa ；when the content of fine slag is 25%100%, the unconfined compressive strength of stabilized soil reaches 115MPa at 90d，much higher than that of cement alone； and the strength of the stabilized soil is positively related to the content of hardened materials，However, it is not closely related to the loss on ignition. The SEM image of specimen for 28d shows that the hydrates of composite stabilizer are well combined with the saline soil and the compactness of stabilized soil with composite stabilizer is much better than that for simply stabilizer. Hardened materials in stabilized soil is the main source of strength of stabilized soil, and it is composed of hydration products of the stabilizer and soil particles bonded by hydration products. The amount of hardened materials is affected by the amount of hydration products and agglomeration.