Abstract: With the extraction influence coal seam 5 # on longwall panel Y485 in Kailuan Tangshan coal mine as research background and based on the key strata theory, the pressure-relief mechanism of lower seam extraction after the upper seam extraction has been studied through numerical model and field ob- servation．The results have shown that during mining, the distribution of abutment stress is influenced by the key strata in the overlying strata．The key strata were fractured due to pressure-relief mining, and the abutment stress was only influenced by the key strata between the two seams．Therefore, the influ- ence distance of the abutment stress and the distance between the peak abutment stress and the panel were decreased．Due to the extraction of seam 5 # in Tangshan coal mine, the abutment stress in panel Y485 located in seam 9 # was only influenced by the key strata between the two seams．The influence distance of the abutment stress in panel Y485 decreased from 73 m to 38 m, and the distance between the peak abutment stress and the panel decreased from 29 m to 20.5 m．When there is a hard-and-thick key stratum present between two seams, the peak abutment stress is 2.34 times as great as that without one and severe stress concentration is formed during the lower seam extraction after pressure-relief mining.

Key words: deep mining, pressure-relief mining, key strata, abutment stress