The natural seawater was used to carry out the experiment of microbial culture and microbial-induced calcium carbonate precipitation (MICP) to solidify calcareous sand. First, the effect of seawater on microbes was studied through the growth and reproduction of microbes and the change of urease activity. Then, the effect of seawater on the solidifying effect of MICP was evaluated according to the changes of calcareous sand permeability and unconfined compressive strength (UCS) before and after MICP treatment. Finally, the mechanism of the effect of seawater on the solidifying of calcareous sand by MICP was analyzed by SEM and XRD tests. The results show that, 1) natural seawater leads to a lag in the growth of microbes, but the number of microbes and urease activity in the stable stage are not significantly different from those in fresh water environment; 2) the effect of using seawater to solidify calcareous sand by MICP is less different than that under fresh water condition, the permeability coefficient of calcareous sand can be reduced by an order of magnitude, and the UCS value can reach 1.7 MPa; 3) under the condition of natural seawater, the MICP process is controlled by seawater composition, microbes, calcium ion concentration, urea concentration and pH. The main crystal form of calcium carbonate is calcite, which can fill the intergranular pores and make the sand particles cement as a whole, which is the main reason for the improvement of mechanical properties of calcareous sand.