Numerical simulation was conducted by using UDEC for the pressure relief effects of protective layer mining in different mining height. The variational law of stress and displacement were gotten. The results show that, after mined the upper protective layer, the vertical stress of protected layer under the gob decreases with increase of mining height, but the vertical displacement of protected layer under the coal pillar increases. This research provide some theoretical basis on the prevention of gas outburst, the drainage system of pressure relief gas is optimized, drainage concentration, volume and rate of pressure relief gas are improved.