In the design of gravel packing sand control, the reasonable selection of gravel size is one of the keys to implementing sand control measures successfully. Aiming at the defects of commonly used methods of gravel size design and the characteristic that the gravel used in field operation is actually a mixture of gravel with multiple grain diameters, this paper builds a model of pore structure in gravel layer through researching the gravel pack structure caused by the gravel of two grain diameters mixed under actual packing conditions, calculates and analyzes the pore sizes in gravel layer. Ultimately, based on Saucier method, this paper presents a new gravel size optimization idea for gravel packing sand control with multiple grain diameters mixed, which agrees with the actual situation of industrial gravel, and gives the idea’s computing method. Considering the ideality of the model in this paper, the author has modified the computing method to make it more fit for the actual packing situation. This gravel size design method also gives consideration to the impact of formation sand uniformity on sand control effect, so it have the characteristics of good practicability, wide applicability and more accurate than other conventional methods.