通过建立煤层开采后顶板初次垮落前后的力学模型，分析了偏态垮落的机理，并推导出开切眼侧与开采侧垮落角的比较式，说明在开切眼侧的垮落角大于开采侧的垮落角，从而在理论上证明了偏态垮落的存在，并揭示了偏态垮落是导致地表下沉盆地偏态的根本原因． Through the establishment of mechanical model before and after the first roof caving after coal seam mining, the mechanism of the collapse of the caving is analyzed and the comparison formula of the caving angle on the cut side and the mining side is deduced. It is shown that the caving angle Which is larger than the caving angle of the mining side, which proves the existence of the caving collapse in theory and reveals that the collapse of the sag is the fundamental reason of the deviation of the surface subsidence basin.