Abstract: The accumulation of excess pore pressure in coral sand under seismic loading until liquefaction is a key factor leading to structural damage. A series of undrained cyclic triaxial tests were conducted in this study to investigate the effects of geogrid reinforcement layer, relative density (D_r) and cyclic stress ratio (CSR) on the development of excess pore pressure and axial strain in reinforced coral sand. The results indicate that geogrid reinforcement as well as an increase in the number of geogrid layers reduce the development rate of excess pore pressure and axial strain, thereby improving the liquefaction resistance of coral sand. The pore pressure of coral sand is much higher than that of siliceous sand under the same cyclic vibration ratio, and the pore pressure development curve of reinforced coral sand gradually transitions from an S-type to a hyperbolic type with the increase of cyclic stress ratio, thus the classic Seed pore pressure stress model is difficult to describe its pore pressure development trend. Based on the above findings, a strain-based excess pore pressure development model for geogrid-reinforced coral sand is proposed. This model accurately predicts the development trend of excess pore pressure in reinforced coral sand under different D_r and CSR, which provides a theoretical basis for the seismic design of infrastructure and stability analysis using effective stress in coral sand island reef area of the South China Sea.