Abstract: In view of the important role of fracture development laws in safety evaluation during coal seam mining under the water body, based on the engineering background of full-mechanized caving mining under the reservoir, by the key strata theory, the combined rock beam theory, and similar physical simulation test method, we studied the breaking laws of overburden rock and the vertical displacement of rock strata, the development height of water conducting fractured zone and the evolution laws of overlying fissure network in the process of rock forming and critical layer breaking. The research result shows that: before and after the collapse of the main key stratum, the subsidence curve slope of observation points in the middle position of gob is larger, and the overburden stratum sinks in the vertical direction shapely; water conducting fractured zone height curve steps up with the periodic rupturing of combimed rock beams, and two steps spacing is the cycle of overburden rock rupture step distance; after the main key stratum breaking down, overburden rock water conducting fractured zone finally reaches the bottom of the inferior key stratum Ⅱ. Fracture network enrichment region mainly focused on the front and rear coal walls, and the fracture density in the crack area near the open-cut hole is greater than that in the lateral fissure of the stop line. Due to the fracture compaction of the fractured rock strata in the middle of the goaf, the fracture density curve of the model is skewed to the “saddle”.