The frequent occurrence of remote landslide disasters under heavy rainfall in the mountainous area with sand-mudstone strata in southwest China is a critical issue to be solved in disaster prevention and mitigation. Taking the July 13, 2020 Niuerwan landslide in Wulong, Chongqing as an example, technical means including UAV image, field investigation, geological condition analysis, and PFC3D simulation were used to study the long-runout motion model of flowslide under heavy rainfall. The results show that the unique stratigraphic structure (Quaternary residual slope soil in the upper part and sand-mudstone in the lower part) is the root cause of the landslide instability and long-runout fluidization movement. Heavy rainfall is the key factor in causing the deep destabilization and overall decline of the landslide, and it also leads to the long-distance movement of the upper saturated residual soil. The long-runout fluidization disaster model of bedding landslide shows the characteristics of overall sliding of the lower layer, mixing of coarse and fine particles of the middle layer, and saturation fluidization in the upper layer. The long-runout fluidization process can be divided into three stages: the overall instability, the mixed acceleration, and the fluidization accumulation. Based on the above research, it is concluded that the investigation and prediction process of long-runout fluidization landslide in the mountainous area with sand-mudstone strata should be based on this particular disaster model to provide a quantitative scientific basis for disaster prevention and mitigation.