Abstract: The islands and reefs in South China Sea are an important support for China's "One Belt and One Road Initiative". As the main building material and foundation of construction on the islands and reefs, the calcareous sand has the bad characteristics of high porosity, easy breakage and low strength, so it is difficult to directly meet the requirements of engineering construction. In order to improve the mechanical behaviors of the calcareous sand and reduce the erosion problems caused by extreme rainstorms, an eco-friendly, cost-effective microbial-induced calcium carbonate precipitation (MICP) synergistic fiber reinforcement (FR) modification technology is proposed. The superficial layer of the calcareous sand is modified by spraying process, and the influences of the cementing liquid concentrations (0.5, 1.0, 1.5, 2.0 mol/L) and the fiber contents (0.1, 0.2, 0.3, 0.4%) on the treatment effect are considered. The cementation degree and surface erosion resistance of the calcareous sand under different treatment methods are analyzed by carrying out mini-penetration tests and simulated rainstorm scouring tests combined with electrical conductivity. The results show that: (1) The MICP technology can effectively cement the calcareous sand and improve its mechanical behaviors. The higher the cementing liquid concentration, the higher the calcium carbonate content and the penetration resistance of the samples, and the curing effect with the concentration of 2.0 mol /L is the best; (2) The addition of fiber can significantly improve the cementing effect of MICP, and the fiber content has an important influence on the mechanical behaviors of the microbial solidified samples. The peak penetration resistance first increases and then decreases with the fiber content, and the optimal fiber content is 0.2%; (3) The MICP-treated samples show better erosion resistance under the simulated rainstorm condition, the erosion amount is less than 1/7 of the untreated samples, and the effect is better after adding fiber. MICP-FR synergy can effectively improve the engineering properties of the calcareous sand and play an active role in the island construction and coastal development.