In underground drilling for drainage in coal mines, the deviation between the drilled borehole trajectory and the designed borehole trajectory is large due to absence of inclination measurement technology to control the drilling trajectory; thus it can not meet the design requirements of gas drainage in coal mines. This paper expounds the technology of three-dimensional trajectory measurement while drilling, aiming at precisely controlling the trajectory of gas drilling in bottom drainage roadways through the investigation of 3-D trajectory measurement technology while drilling, and achieving fast tunneling, and safe and efficient mining in gas outburst coal seams. The technology of 3-Dimensional trajectory measurement while drilling is to accurately measure the inclination of drilling holes and the trajectory of drilling holes, and present them with the 3-D trajectory mapping method. Through a large number of data acquisition and field verification, it has been proved that this method has become an important link in the process of coal seam safety and production, avoiding the blindness in borehole design and drilling, and improving borehole drainage coverage and drilling rates.