Abstract: In order to reveal a relationship between a deep- 850 m gas dynamic phenomenon and the geological control factors in Qianjiaying Mine,based on the deep gas measured data of the mine,the paper studied the seam thickness,mine structure,geostress,gas pressure and content, and other geological control factors. The results s howed that a thickness of No. 5 seam in the gas outburst area was abnormal and the variation coefficient of the thickness was higher than 90%. Many reverse faults made the seam structure more broken,the geostress was high and the horizontal main stress was 43.87 MPa in max. In gas outburst area,the gas content would be high and the gas pressure measured in the outburst area reached at 4. 6 MPa. Based on the above results,according to the' Regulations of the coal and gas outburst prevention" ,the gas pressure critical value was 0. 74 MPa. In the fold development area,structure simple area and fault development area,the variation coefficient of different seam would be se lected as a threshold. A danger grade division was conducted on the gas dynamic of No. 5 seam. The division results showed that there were two gas outburst danger area s in the north part of the mine and other area of the mine was an important prevention area of the gas outburst.