Abstract: Stacking using flexible intermediate bulk containers (FIBCs), commonly known as ton bags, is a widely adopted method for storing packaged finished grain. However, due to the considerable height of such stacks, they are prone to collapse, posing significant safety risks to personnel and compromising grain storage integrity. This study investigates the collapse mechanisms and stability of FIBC-stacked grain through field collapse experiments. Tests were conducted on four stacking configurations: a two-layer stack, a three-layer stack with uniform interlayer offsets, a three-layer stack with varying interlayer offsets, and a three-layer stack where the third layer remained unoffset. The influence of FIBC flexibility on the collapse process was examined, and theoretical calculations and mechanistic analyses were performed. The results demonstrate that: The theoretical model assuming a rigid-body stack yielded collapse prediction errors of 45.5%-63.6% compared to experimental data. The collapse mode follows an "offset-deformation" positive feedback loop, driven by the reduction in effective contact area between layers and interlayer contact deformation. A dual-parameter predictive model incorporating the effective contact area coefficient (β) and the interlayer contact deformation coefficient (γ) was developed to account for FIBC flexibility. Using quantified values of β=0.82 and γ=1.87, the prediction error was reduced to 5.0%-12.0%. This model provides a reliable basis for the safe design and stability evaluation of FIBC-based grain storage systems.