The use of moving granular bed filters (MGBFs) had being growing, and they held an important role in the gasification or combustion of coal or biomass. Furthermore, they had great potential to be developed for the high-temperature gas cleanup of advanced power generation systems. However, the existence of a stagnant zone with defective designs in MGBFs could cause serious damages such as plugging. Therefore, in order to minimize these flow problems, flow-corrective inserts for filter vessels had been researched. Most previous theses used mono-sized filter granules as filter media in a filter vessel. Meanwhile, this study proposed a new method that introduced two filter granule sizes in one filter vessel, called the two-stage filtration mode. Nevertheless, the design theory of a mass flow vessel by Johanson could not be satisfied by using two sizes of filter granules to diminish stagnant zone. The six test conditions of configuration to accomplish the objective were explored in this study. The results included the flow patterns in a two-dimensional and cross-flow moving granular bed with the different geometry designs in a filter vessel. The two kinds of filter granules consisted of coarse and fine silica sands. The findings revealed that the flow pattern profiles of filter granules were influenced by the vessel geometry. The optimal design for two-stage filter granules in a filter vessel was created to diminish the stagnant zone in 165 minutes.