Abstract: To address the insufficient understanding of floor stress propagation under intense mining-induced disturbance in deep, extra-thick coal seams, the 40205 fully mechanized top-coal caving working face at Xiaozhuang Coal Mine, operated by Shaanxi Binchang Xiaozhuang Mining Co., Ltd., was selected as the engineering background. Similar material simulation, field measurements, and theoretical analysis were integrated to systematically investigate floor stress propagation governed by the fracture of an extra-thick key stratum. The results indicate that floor stress exhibits a staged evolution with the advance of the working face, while floor strain displays a three-zone distribution pattern consisting of a stable zone, a concentration zone, and another stable zone. Furthermore, the fracture of the high-level key stratum induces pronounced fluctuations in floor strain. Field measurements reveal that periodic weighting at the 40205 working face is characterized by alternating major and minor weighting cycles, consistent with the hierarchical fracture behavior of key strata observed in the physical simulation. Theoretical analysis further demonstrates that, during major periodic weighting, the maximum vertical stress increases by 108% compared with that during relatively stable periods, accompanied by an increase in the stress concentration factor to 3.0–3.2 and an extension of the high-stress influence zone to a depth of 18 m. These findings elucidate the mechanical mechanism whereby the fracture of a high-level sub-key stratum alters the load transfer path of the overburden, resulting in the displacement of the stress shell within the surrounding rock and consequently intensifying floor stress disturbance and its downward propagation. The findings provide a theoretical basis for predicting the failure depth of floor aquicludes and optimizing roadway placement horizons under intense mining-induced disturbance.