Abstract: Fluid mud is widely distributed in coastal areas, estuaries, and reservoirs, exerting significant impacts on regional ecology, the environment, and navigation. The characteristics of fluid mud vary depending on its formation mechanisms. To quantitatively analyze the effects of sediment flocculation on the density and rheological properties of fluid mud, a large-scale tank experiment was conducted to simulate flocculation and deposition processes of cohesive sediment. Bed surface deposits were collected for rheological testing. The results show that as floc size decreases and fractal dimension increases under turbulent conditions, the density of the resulting fluid mud increases, while its yield stress and viscosity coefficient exhibit a power-law growth. The presence of interfloc pores reduces the structural strength of the deposited fluid mud, causing the rheological curve to lack a distinct solid-liquid transition stage and instead undergoes direct yield deformation at a critical point, entering the liquid phase. When the influence of sediment flocculation is considered, fluid mud with a density greater than 1,110 kg/m³ exhibits a marked decrease in yield stress and viscosity coefficient, accompanied by an increase in flow index. These macroscopic rheological variations are governed by microscopic particle interactions, particularly the strength and quantity of contact points between sediment particles. Flocculation alters both particle structure and interparticle forces, thereby influencing the macroscopic rheological behavior. This study provides valuable insights for the numerical simulation of fluid mud transport processes.