Recently high-frequency sudden pollution originated from an explosion of industrial plants, transportation accidents and oil spills was often continuously diffused into downstream water. In this paper, the features of pollution diffusion including the boundary shape, concentration gradients and covered area from the two typical sudden water accidents of river mouth inflow (RMI) and beach uniform inflow (BUI) were investigated and compared by utilizing computational fluid dynamic (CFD) model and lab-scale experiments. It was found that a circular boundary shape was formed when diffusion velocity was slower than threshold velocity of 0.0016 m/s, however, a long strip of boundary was replaced at the speed of more than 0.0016 m/s from RMI, using CFD simulation under lab-scale. The coincidence degree of diffusion over time in terms of covered area and boundary shape between CFD simulation and the lab-scale experiment was reached to 97.6–99.6% both in RMI and BUI. The result indicated that CFD was applied to simulate the pollution diffusion from the two patterns of sudden water accidents under full-scale. Results showed that a sharp peak was capable of appearing in mainstream and there was a ring current appearing in side wing from RMI. However, the mainstream with a gentle peak and the side wing with symmetrical diffusion were arising in BUI. In addition, a high concentration gradients and a clear concentration contours were both exhibited in RMI and BUI. The results may assist in offering emergency response to control sudden pollution diffusion, further supporting the scope of pollution hazard assessment and ecological remediation to recover pollution region.