The reserved thickness of top coal has an important influence on the stability of a large section open-off cut under gob in the thick seams slicing mining. The destabilization extremum conditions of the open-off cut top coal were derived from by elastic-plastic theory, and the optical fiber sensing technology was utilized to monitor the top coal deformation law with different thicknesses (3, 3.5, and 4m) in the physical similar simulation experiment in the paper. The results show that the top coal thickness is greater than 3.4m without tension cracks. In the vertical direction, the top coal of the LSOOC is divided into mining and excavation disturbance zones under the influence of the upper slice coal mining and the excavation disturbance. In the direction of the span of the open-off cut, the top coal can be divided into the roof fall risk zone and the warning zone. The deformation changes from exponential to linear to logarithmic in the roof fall risk zone, and it changes from linear to logarithmic in the roof fall warning zone as the number of excavations increases. The sinking amount in the two zones is smaller as the thickness of the top coal becomes larger. It is comprehensively determined that the thickness of the top coal of open-off cut is set as 3.5m, the stability is moderate, and the field application shows that the integrity of the top coal is good after support, and the maximum off-layer value is 6mm, which can satisfy the production requirements.