Abstract: For the project case of mining under thick hard igneous roof, through theoretical analysis of the formation mechanism and conditions of separated stratum space, we discriminate “cavity type” separation location of the first face in study area. The process simulation and dynamic monitoring analysis are adopted to simulate the characteristics of the evolution of separation stratum space and deformation of overburden strata through numerical method and optical fiber monitoring technology. Research shows that: a total of 5 roof rock at the first primary face can develop the “cavity type” separation stratum space. The movement of overburden hard strata can also be divided into four steps which are named lower support, extension of the bottom separated stratum space, instability movment and closure of separated stratum space. The optical fiber monitoring value of tensile stress in separated stratum location increases gradually. The separation stratum develops gradually from the lower strata to upper strata until the upper igneous rock when the overburden strata of separation stratum loses stability by its self-weight stress. The results are consistent with the numerical simulation and theoretical discrimination.