In order to determine the mixing proportions of slurry during slurry shield tunneling under seawater formation environment, the laboratory tests, including performance comparison test between fresh water slurry and seawater slurry，seawater infiltrating fresh water slurry test, different mixing proportion test with additives and solid phase materials, and filter cake formation test of fresh water slurry under seawater formation environment, are conducted. The test results show that: (1) The appropriate bentonite proportion of fresh water slurry without additive should be 18% to 24%，and the viscosity can be reduced by adding carbonate for slurry concentration of 30% . (2) With the increase of bentonite content, the effect of seawater infiltration capacities on slurry colloid fraction decreases and that on viscosity increases. (3) Hydroxy propyl methyl cellulose (HPMC) can increase the seawater slurry colloid fraction and the slurry viscosity, and reduce the segregation rate. (4) When the solid phase ratio is fixed, the addition of clay can solve the problem of excessive viscosity caused by reaction of bentonite and HPMC, but will also increase the filter loss of seawater slurry. (5) The filmforming effect of fresh water slurry with bentonite content of 18% under seawater environment of Su′ai Tunnel is good.