Fragile geomaterials may affect the real results of the gradation test by particle breakage, which further affects the analysis of its engineering properties and the engineering construction. In order to explore the gradation test method suitable for fragile geomaterials in different working conditions, different gradation test methods were studied with coral sands as the test object to meet the needs of engineering construction and operation. The results show that the vibrating and sieving time has a great influence on the gradation of coral sands. The characteristic particle diameters of coral sands also change most greatly in the first 25 min of vibrating and sieving. The non-uniformity coefficient Cu and curvature coefficient Cc are changed the most in the first 10 min of vibrating and sieving, and then the non-uniformity coefficient Cu continues to increase, but the curvature coefficient Cc is basically unchanged. For the constructions that need to evaluate the coral sands breakage caused by external loads, such as filling projects and foundation treatment, it is recommended to use the vibrating and screening to obtain its particle composition and gradation characteristics, and the vibrating and screening time should not be less than 25 min. The cumulative quality of coral sands has a good correspondence with the cumulative planar projected area. Therefore, the cumulative planar projected area of coral sands can be used to determine its gradation characteristics, and the test results can be used to evaluate the engineering properties of coral sands.