Abstract: Based on an engineering qualitative accident instance of composite foundation with long and short piles, the field investigation and foundation deformation monitoring are conducted. The causes of engineering qualitative accident are analyzed according to the load transfer mechanism of composite foundations with long and short piles under flexible foundation. The causes of engineering qualitative accident are found, and the relevant precautionary measures are proposed. The analysis results show that there are two different failure modes in the improved ground foundation because of the soil consolidation and settlement due to the large-area loading. If the pile tip is suspended in the soft soil layer, it will penetrate into soft soil at the first failure stage, then plastic failure will occur in the soft soil among piles, as a result, the foundation treatment may fail. If the pile tip is embedded into the hardpan layer, the pile body will firstly be destroyed and then the soft soil among piles will be destroyed. The primary cause of the above engineering qualitative accident is the consolidation settlement of the soil around piles under large-area loading and the pile tip suspending in the soft soil layer. The research results may provide reference for similar engineering and associated theoretical researches.