Abstract: The No. 418 working face of Chenjiashan Coal Mine is a typical fully mechanized caving working face in a thick coal seam, and the main mined coal seam is No. 4^-2 coal seam, with an average thickness of 8.0 m. In order to obtain accurate data on the development height of the mining-induced overburden fractures, this study designed and adopted the distributed optical fiber monitoring technology, and carried out the actual measurement work by arranging two holes underground. The research results show that the development height of the water-conducting fracture zone in the overburden of the roof of the No. 4^-2 coal seam is 112.5 m, and the development height of the caving zone is about 41.7 m. The overburden fracture zone finally develops to the bottom of the extremely thick sandstone, which fully indicates that under the structural conditions of the extremely thick sandstone layer, the development height of the fracture zone in the roof of the coal seam is significantly inhibited by the extremely thick sandstone layer. The test results provide a basis and reference for the water prevention and control design of Chenjiashan Coal Mine and its adjacent mines, and provide a technical guarantee for the safe production of mines.