Abstract: In order to study the influence of key stratum location on pressure relief minnig effect, by using numerical simulation software UDEC, we investigated the dynamic evolution and stress distribution and swelling deformation of protected seam during the process of coal seam mining at lower protective seam. The result indicates that when the key stratum s between the protective seam and protected seam, the protected seam is located in bend belt with less fissures; when the key stratum is above the protected seam, overburden rock fracture evolves to the protected seam, which is better for the development of micro fissures and provides favorable conditions for gas extraction; the influence of key stratum fracture of different locations on pressure relief effect is opposite; the position of key stratum has big influence on swelling deformation of protected seam; the deformation of protected seam is influenced by key stratum fracture; pressure relief of protected seam has obvious time-space effect due to the different key strata position.