Abstract: According to the thick hard conditions of separated stratum space under overlying high-position magmatic rocks in working surface, the evolution rules and morphological characteristics of separated stratum space under the main key strata are studied by similar material simulation test. The horizon evolution and horizontal scaling law of separated stratum space and the migration law of overlying strata and the main key strata are analyzed. Through theoretical analysis, the formation mechanism and conditions of separated stratum space are revealed. A method to determine the evolution characteristics of separated strata space is put forward, and the analysis model and prediction method of the maximum separated stratum space at the bottom of main key strata are proposed. Researches show that the instability migration of combined strata forms a "crescent moon" shaped separated stratum space which intermittently jumps up to the bottom of the magmatic rocks. When the separated stratum space densely exists at the bottom of the magmatic rocks, and is gradually transformed from the " crescent " form into the “straight line " one, the burst migration of magmatic rocks contributes to the closure of separated strata. The separated stratum space lies at the bottom of the inferior key stratum and the main key stratum dynamically developes and closes from bottom to top. There is a positive correlation between the layer heights together with the development scope and the advancing distance. The “multi-trapezium” method can be used to determine its evolution process. The migration of overlying strata experiences five-state subsidence of the inferior key stratum, development migration to the bottom of main key stratum, migration of the magmatic rocks and the overall stability. The subsidence form of the overlying strata is successively presented as shapes of “V”, “√” and “U”. The migration of magmatic rocks can also be divided into five steps which are named lower support, extension of the bottom separated stratum space, rupture of the follow-up, instability migration and stable closing of separated stratum space. The overlying strata structure of the maximum separated stratum space and the elastic foundation beam model are established, and the forecast formula for its area and volume is derived so as to provide theoretical basis for the safety mining under high magmatic rocks.