In order to explore the hydraulic performance of the screen filter, the initial flow field inside the screen filter and the flow distribution on the mesh surface of the filter element were fully understood. Computational fluid dynamics method was used at three entrance velocities (0.5m/s, 1.5m/s and 2.5m/s)and three kinds of mesh numbers (60meshes, 80meshes and 100 meshes) for numerical simulation, and the screen filter head loss was analyzed. The head loss of the filter was concentrated on the outlet core, which accounted for 87% of the total loss, and increased with the improvement of mesh number of filter screen. Stream flowed to the lateral acceleration area inside the cavity can be divided into export, export side deceleration zone, plug circumfluence area and vortex area four parts. The maximum velocity in the cavity was located at the end of the accelerating zone on the outlet side, and the difference between the maximum velocity and the inlet velocity was 2.4 times. The mesh number of filter screen did not affect the distribution of internal velocity field. The flow distribution on the screen surface was seriously uneven. The high-flow area was mainly distributed on the outlet side, and the area of the high-flow area was positively correlated with the inlet velocity and filtration accuracy. It was suggested to block the area with the highest and lowest flow on the net surface, so that the flow distribution on the net surface can become even, and thus improve the filtration efficiency. When the inlet velocity was increased from 0.5m/s to 2.5m/s, the difference between the maximum and minimum flow was 3.3 times. When the mesh number of filter screen was increased from 60 meshes to 100 meshes, the difference between the maximum and minimum flow rate was 3.3, 3.1 and 2.3 times, the maximum flow rate was shifted outward, and the minimum flow rate was shifted inward. The pressure field in the cavity was step-down with the vortex area as the center. The distribution law of pressure field did not change with inlet velocity and mesh number, but the pressure field was positively correlated with the two variables. In order to reduce the impact force of water flow on the filter screen, annular plates can be added on the surface of the filter screen to form a stagnant water area so as to reduce the impact of water flow on the outlet side of the filter screen. The dead water area in the plug had high pressure, low flow rate, and sediment was easy to precipitate. It was suggested to expand and optimize the plug volume to undertake more sediment. The research result can further reveal the changing rule of the flow field inside the screen filter, and explore the mesh flow distribution of the filter, so as to provide a reference for the structure optimization of the filter.