Coral sand (CSD) was mixed with fine soil in varying proportions within real reef ground, exhibiting intricate dynamic behaviors under cyclic loadings, such as earthquakes. Undrained cyclic triaxial tests under diverse confining pressures and cyclic loading stress ratios (CLSR) with various fines content (FC) were conducted to systematically investigate the liquefaction characteristics and excess pore water pressure (EPWP) development. The coarse-grained CSD and fine-grained CSD displayed distinct failure modes, with the fine-grained CSD exhibiting a greater propensity for liquefaction. The liquefaction resistance of CRR₂₀ (cyclic resistance ratio) showed a trend of first decreasing and then increasing with the increase of FC, and reached the minimum value near the critical FC (FC_th). As FC and confining pressure increased, the axial deformation transformed from a gradual accumulation mode to a slow-rapid development mode. Particle breakage decreased with increase of FC but increased with confining pressure and CLSR. The plastic exhibited a trend of initial decrease followed by subsequent increase as FC rose, attaining its minimum value around FC_th. EPWP development curve changed from Type A to Type B as FC and confining pressure increased. EPWP exhibited differential responses during the third stage of Type B development pattern, when FC was relatively low (coarse-grained CSD), the growth rate of EPWP slowed down; while when FC was relatively high (fine-grained CSD), it continued increasing. The EPWP development of CSD mixtures exhibited notable discrepancy from that of quartz sand, thus a new unified EPWP model was proposed. The new findings obtained in this paper provided valuable insights for seismic design of infrastructure on coral island.