Waterweed is one of the key elements in Eriocheir sinensis （Chinese mitten crab） pond aquaculture. In order to investigate the mutual influence and mechanism of waterweed clump planting offset and pond flow field in aquaculture ponds， this paper conducts a numerical simulation study on the flow field characteristics of waterweed clump offset based on porous media. Using the solid volume fraction of waterweed clump as a clue， a set of porous media experimental devices are established to quantify the resistance of waterweed. A numerical model of waterweed clump is established based on field measurement samples， and the influence of waterweed clump offset on the pond flow field is studied through 5 sets of incremental offset experiments in Fluent. The study shows that increasing the offset can significantly change the flow velocity distribution from strip-like to patchy， promoting lateral mixing of water. The index of flow uniformity fluctuates around 0.6， and the medium offset has the highest uniformity of water flow. The velocity and uniformity inside the waterweed clump increase with the increase of offset， mainly depending on the area where the secondary waterweed clump leaves the shear zone of the first row of waterweed clumps. The energy utilization rate of the highest offset condition can reach 0.49， 4.6 times the lowest value. The vertical vortex behind the waterweed clump is opposite to the change trend of the boundary layer separation vortex. A high offset will weaken the range of action of the vertical vortex and enhance the range and intensity of the boundary layer separation vortex. The research results help to understand the energy circulation and material distribution in crab ponds， and scientifically guide the planning of waterweed planting in crab ponds.