Abstract: The particle morphology is an important microscopic characteristic that affects the physical and mechanical properties of granular soils. Carbonate soil particles have complex morphology due to special biogenesis. In order to study the morphological characteristics of the soil particles, the PartAn 3D particle dynamic image analyzer was used to test the particle shape of the carbonate soil with grain sizes of 0.5 ~ 20 mm from the South China Sea. The elongation, flatness, sphericity, roundness, angularity and convexity were used to quantitatively describe the morphological characteristics of the particles. The results show that the frequency distribution of the elongation, flatness, sphericity and roundness of carbonate soil particles in the South China Sea complies with the normal one, while the frequency distribution of the angularity and convexity complies with that of the power law. The particle shape of the carbonate soil is mostly blocky, and with the decrease of grain size, the soil particles become flatter and more regular in shape. Through investigating the effects of sample size on the results of particle shape quantification, it is suggested that the number of particles should be not less than 600 when using the arithmetic mean of shape descriptors to quantify the morphological characteristics of the uniformly graded carbonate soil. Finally, a comprehensive shape index which can fully describe the morphological characteristics of the carbonate soil particles is obtained with the aid of the principal component analysis method in statistics, and the relationship between this shape index and the maximum to minimum void ratio of the carbonate soil is established.