Coral calcareous sand, a unique geotechnical material primarily composed of calcium carbonate, is widely utilized in military and civil construction on island reefs. However, research on the mechanical properties and micro-mechanisms of gypsum cemented calcareous sand remains insufficient. The unconfined compressive strength test was used to investigate the changes of mechanical property of gypsum cemented calcareous sand samples. By scanning electron microscope test, the evolution laws of the sample's micromorphology was analyzed, and the microscopic mechanism of mechanical property of gypsum cemented calcareous sand was revealed. The results show that under the same water-gypsum ratio, the unconfined compressive strength of gypsum cemented calcareous sand samples increases with the gypsum content. Under the same gypsum content, the water-cement ratio of the samples with low gypsum content (5% and 10%) increases with the increase of water-cement ratio, and the water-cement ratio of the samples with high gypsum content (15%, 20% and 25%) increased first and then decreased with the increase of water-cement ratio. The optimum ratio of gypsum cemented calcareous sand samples is gypsum content being 25% and water-cement ratio being 0.9. The fundamental reason for the above changes in mechanical property is that building gypsum (the main component is semi-hydrated gypsum) reacts with water and transforms into dehydrated gypsum, which connects the calcareous sand particles in the samples and enhances the cementation between the soil particles.