Abstract: Due to the limitation of the number of samples, the particle-size envelope method for directly determining the sand-gravel design line according to the gradation curve of exploration stockyard is difficult to reasonably reflect the actual gradation range of the sand-gravel materials in the material field, resulting in some of the filling gradations being unqualified, so that it will affect the assessment of dam filling quality. For this reason, based on the construction materials of Aertash, Dashixia and Dashimen, the suitability of the Weibull model to reflect the convex, S-shaped, or inverted S-shaped gravel materials and the gradation characteristics of the fractal model is demonstrated. The gradation of gravel materials in the stockyard of the Aertash, Dashixia and Dashimen is divided into two different types: S-type and convex-type. For the first time, the method of using the maximal particle size to standardize the gradation to determine the grading of the material yard is proposed. It can make up for the shortcomings that the grading package cannot objectively reflect the gradation range of the yard due to insufficient exploration data. The research results can be directly used for the gradation design of gravel materials, and can also be used to determine the envelope of different types of soil gradations, which shows a strong application value.