Abstract: In order to get actual gas content of underground coal mine during large-scale advanced detection, the sealed coring method for gas content was proposed. The sealed coring device of "three-cylinders, single motion and ball valve sealing" suitable for accurate test of gas content of long boreholes in underground coal mines was designed and trial-produced, the outer diameter of the device is 89 mm, and the diameter of the coal core can reach 38 mm, the pressure holding capacity of the sealed coring device is above 11 MPa, which satisfies the requirement of sealing sampling for the determination of coal seam gas content in underground coal mines. By using the sealed coring device to carry out the test of gas content determination of underground coal coring in typical coal mines in China, the depth of coal coring in coal coring reached 360.5 m. The coal seam gas content measured by 5 groups of sealed coring method was 1.26~3.06 times higher than that of conventional coring method, with an average of 2.25 times. This technology will provide an effective technical means for test of the effect of extensive accurate detection of gas content and regional gas control in underground coal mines in China.