The slurry degradation induced by seawater and saltbearing formation often occurs to slurry shield boring undersea. As a result, the difference among the slurry formulated by freshwater, seawater and NaCl solution is analyzed by the swelling deformation test based on the subsea tunnel project from Wuyuanwan Station to Liuwudian Station on Xiamen Metro Line 3, and the rule of the property variation of bentonite slurry under seawater intrusion is analyzed by seawater infiltrating freshwater slurry test. The experimental results show that: (1) Slurry formulated by seawater or NaCl solution will significantly reduce the Zeta potential of the bentonite slurry, resulting in poor swelling of the bentonite and even expansibility damage. (2) Under seawater intrusion, the slurry, of which the quality of sodium bentonite is one tenth of that of freshwater, particles and the water separates when the amount of seawater intrusion increases to 10% of the slurry quality. (3) The injection of additives such as CMC and HS3 into the slurry is beneficial to the stability improvement, resulting in slurry viscosity increase and bleeding rate reduction of slurry with seawater intrusion. By using the composite slurry modified by CMC and HS3, the Wuyuanwan Station to Liuwudian Station Section on Xiamen Metro Line 3 has been successfully passed through the waterrich and high permeable stratum.