Abstract: In order to improve the permeability of coal seams and enhance the underground gas drainage，the combined gas drainage technology of surface powerful hydraulic fracturing and underground gas drainage was adopted．Through the analysis on the distribution characteristics of the strata stress and the fracturing law of the coal seam under water pressure，two typical shapes of hydraulic fractures were given，the displacement field of the fractures under the conditions of the plane stress was calculated，and the theoretical calculation formula of the fracture radius based on the highly elliptical fracturing shape of the fracture was obtained，this provided the basis for the surface fracturing design．Also，the fracturing technology system for surface fracturing and underground drainage was proposed and the location principle of fracturing was put forward．The surface fracturing tests were carried out in Sihe Coal Mine，the test results showed that the drainage quantity was 3．65 times higher than the original hole after permeability improvement by fracturing，the accumulated gas drainage quantity by fracturing holes was 2 186 600 m³，the fracture radius was 120 m，and the better permeability improvement effect by fracturing was achieved．