Abstract: When a fully-mechanized upper slice mining was conducted in the bottom slice of the extreme thick seam in Lingquan Mine, according to the problem occ urred that the gas in the water drainage gateway of the upper slice was easily run into the up gateway of the bottom slice.A borehole drilling was conducted from the up gateway in the bottom slice to the water drainage gateway in the top slice in order to make the nitrogen injection to replace the gas.CFD numerical value was applied to simulate the migration law of gas in the water drainage gateway in the upper slice and the goaf.In order to have the rational nitrogen injection parameters,the variation of gas concentration in the water drainage gateway with different nitrogen injection rate and the nitrogen injection spacing was analyzed.The results showed that when t he parameters of the nitrogen injection varied,the distribution law of gas in the water drainage gateway would have differences.When the borehole space was 30 m and the nitrogen injection amount was 800 m~3/h,the replaced gas effect was best.Finally,the rational nitrogen injection parameters should be applied to the industrial site a nd the results showed that in the bottom slice mining,there was no gas over limit phenomenon occurred.