Abstract：Due to differences in formation time and mineral composition，rock strata contain numerous layers of different thickness and strength. Practical experiences have shown that one or several thick and strong strata have a dominant effect on the movements and subsidence of rock strata above the underground mining excavations. The key stratum is defined as the stratum that controls the movements of a part or all of the strata in the overburden. The former is called the subordinate key stratum，and the latter is called the primary key stratum. By means of comparative analysis on the field measurement data of stratum movement and the surface subsidence，numerical and physical simulation，the influences of the primary key stratum on surface dynamic subsidence are studied. The results prove that the primary key stratum in the overburden control the dynamic process of surface subsidence，the break of the primary key stratum will obviously augment the subsidence speed and the subsidence boundary，and the subsidence speed and the subsidence boundary will change periodically as the primary key stratum break periodically. Based on the key stratum theory in ground control，the basic principle in the design of mining under buildings should ensure that the primary key stratum would not break.
