Abstract: The calcareous sand is widely distributed in offshore continental shelves, coastal zones and oceanic reefs. It has the characteristics of low strength and fragility and has been eroded by wind and waves for long time. In order to improve the mechanical properties of the calcareous sand, a microbial-induced calcium carbonate precipitation (MICP) method is proposed, a series of cementing processes are reproduced in simulated seawater environment, and the unconfined compressive strength of the specimens is tested. At the same time, the results are compared with those obtained in fresh water environment. In addition, the effect and mechanism of urea concentration on the mechanical properties of MICP-treated calcareous sand are investigated by setting cementing solutions with different urea concentrations (0.25, 0.5, 1.0 and 1.5 mol/L). The results show that: (1) The MICP technology can be applied to marine environment, the reinforcement effect of the calcareous sand is better than that of fresh water environment, and the unconfined compressive strength of specimens after solidification in seawater is doubled compared with that in fresh water environment. (2) The alkaline environment of seawater has a positive effect on the activity of urea bacteria and the cementing effect of MICP. (3) The urea concentration has an important impact on the cementing effect of MICP, and the unconfined compressive strength of the specimen first increases and then decreases with the increase of urea concentration. It is found that the optimal urea concentration is 1.0 mol/L. (4) The unconfined compressive strength of MICP-treated specimens is positively correlated with the content of microbial-induced calcium carbonate.