Abstract: In order to study gas migration law in overlying strata cracks after mining of coal seam, gas migration test bench is developed. This test bench solves the problem of pressure relief gas migration law of different coal seam mining heights and ventilation rates. The experimental study of gas migration in the three dimensional experimental platform shows that the gas concentration in initial segment goaf area is low and the trend of rising is slower in the direction of goaf depth; in the depth of goaf, gas concentration continued to increase and tends to be stable; in direction of goaf height, the gas carries out floating movement under the effect of density difference, which leads to the upper goaf gas concentration higher than the bottom of the gas concentration. The experimental results are highly consistent with the field results.