Abstract: To explore the influential laws and degrees of gravel packing solidity on its sand blocking effect and stability when the sand control technologies of gravel packing is carried out, the experimental device for testing the comprehensive performance of sand retention media was applied to radial flow displacement experiment of sand blocking gravel. In the experiment, quartz sand and artificial ceramsite were adopted and four packing solidities were simulated, i.e., 100%, 98%, 96% and 95%. And the sand blocking effect and stability of gravel were evaluated based on permeability, sand passing ratio and erosion morphology change. The following experimental results were obtained. First, perforation inflow can lead to erosion vugs in the gravel, and with the reduction of packing solidity, the area of erosion vug and the invasion depth of formation sand into the gravel are increased. Second, when the packing solidity is lower than 95%, larger vugs are formed and the gravel gets instable. And consequently, the formation sand flows through the gravel completely and the gravel is invalid to sand blocking. Third, as the packing solidity decreases, the unconsolidated particles are rearranged under the effect of fluid impact, and the generated vugs increase the invasion depth of formation sand into the gravel and the sand passing ratio. As a result, the sand blocking effect gets worse. And fourth, to realize the sand control of inside casing gravel packing in casing perforating wells, the packing solidity shall be higher than 95% as much as possible, and the slot width or sand blocking accuracy of mechanical screen shall be designed according to the principle of blocking the formation sand directly.