[Objective] The purposes of this study are i) to conduct the prediction and evaluation of debris flow disasters accompanied with the building spoil ground corresponding to diverse mixed ratio of different spoil under extreme rainfall conditions, and ii) to reveal the chain evolution mechanism and spatiotemporal distribution of the disaster chain of the building spoil ground in mountainous areas under extreme rainfall conditions. The research results provide a scientific basis for the prevention and control of soil erosion in mountainous areas. [Methods] Five cases (denoted as cases 1-5, respectively) which comprises five different spoil composition of miscellaneous fill, silt, and ceramic waste are designed to analyze and simulate debris flow accompanied with the typical building spoil ground. Cases 1-5 involves single-material stacking or mixed stacking with different proportions of spoil compositions. The stabilities of the dam and the slopes formed by spoil, together with the characteristics of debris flow after dam break in spoil ground are systematically analyzed. The coupling dynamic model for landslide and debris flow is established by Massflow software. The hazard zonation evaluation of debris flow is carried out based on the simulation results provided by the model. The sensitivities of the characteristics of debris flow to the key model parameters impacted by the mixed ratios are investigated. [Results] (1) The safety factors for the dam are 1.049, 1.002, and 1.034 when extreme rainfall occurs and the building spoil ground is fully filled, while these values for the filled materials are 1.172, 0.826, and 0.959. These results show that extreme rainfall events may trigger the dam break of the dam. The dam break can induce the instability of the filled material behind the dam, which therefore lead to the formation of debris flow along the downstream channel. (2) The numerical simulation results show that the maximum velocities of debris flow in cases 1-3 are 21.04 m/s, 25.36 m/s, and 18.73 m/s respectively. The maximum mud depths of debris flows are 19.2 m, 8.2 m, and 12.7 m respectively. The farthest accumulation distances of debris flow in cases 1-3 are 356.0 m, 674.8 m, and 545.4 m respectively. The areas of high-hazard zones of debris flows in cases 1-3 are 36068.1 m2, 77254.9 m2, and 82887.0 m2 respectively. (3) A joint analysis of the simulation results of cases 1-5 and the ranking of factors (i.e., internal friction angle > excess pore water pressure coefficient > unit weight) to which the characteristics of debris flow is sensitive reveals that the internal friction angle and excess pore water pressure coefficient, which are impacted by the mixed ratio, are the main controlling parameters for the characteristics of debris flow. [Conclusions] This study shows that under the conditions of full reservoir and extreme rainfall, the building spoil ground in mountainous area is associated with high possibility to cause a disaster chain of dam break, spoil ground destabilization, and debris flow, which poses a serious threat to the safety of downstream residential areas and industrial facilities. The simulation results document that the use of a reasonable proportion of material to perform a mixed landfill scheme can effectively enhance the overall stability of the spoil ground, and significantly reduce the area of high-hazard zones of debris flow. A scientifically mixed landfill scheme can effectively control the influencing scope of debris flow. These research results provide an important reference for the optimal design of soil and water conservation projects for building spoil grounds in mountainous areas.