The horizontal screw decanter centrifuge is an automatic and continuous settlement centrifuge; it is widely used in different industries. Within the centrifuge, the drum and screwpropeller rotate at different speeds in the same direction, and therefore, the fluid flow state is extremely complex. There is no theoretical method to describe the solid concentration distribution during the filtration separation process. Therefore, FLUENT software was applied to an M-2301-type horizontal screw decanter centrifuge to simulate the fluid flow situation in the centrifuge drum and obtain the solid concentration distribution. The simulation results showed that the solid volume fraction could reach 66.35 and 4.92% at the sediment outlet and the overflow outlet after 10 s of simulation, respectively, and it was in good agreement with the industrial experimental results. The research shows that the 3-D unsteady calculation combined with moving mesh is an effective technique to investigate solid concentration distributions in the decanter centrifuge. Furthermore, the input concentration, which affected the solid distribution and flow integrity in the drum, was analyzed. The results show that the horizontal screw decanter centrifuge can effectively handle high solid content suspensions.