The existing deep-sea sediment plume tests are mostly under small-scale static water and rarely under large-scale flowing water conditions. In this study, large-scale tank experiments of flowing water were designed and conducted to investigate the morphological characteristics and concentration evolution of the sediment plumes under different discharge rates (Q) and initial sediment concentrations (c). Viscosity tests, resuspension tests and free settling tests of the sediment solution with different c values were performed to reveal the settling mechanism of the plume diffusion process. The results show that the plume diffusion morphology variation in flowing water has four stages and the plume concentration evolution has three stages. The larger the Q, the smaller the initial incidence angle at the discharge outlet, the larger the diffusion range, the poorer the stability and the more complicated the diffusion morphology. The larger the c, the larger the settling velocity, the faster the formation of high-concentration accumulation zone, the better the stability and the clearer the diffusion boundary. The research results could provide experimental data for assessing the impact of deep-sea mining on the ocean environment.