Disallowed gas concentration (DGC), i.e., a cease in production due to gas concentration exceeding the allowable limits, occurs frequently during the initial mining phase of longwall faces in Yangquan Coalfield, and seriously affects safety. The mechanism of DGC during the initial mining phase of longwall face is determined by analyzing the outflow behavior of methane. The results show that mining can induce ground movement and lead to pressure relief, leading to desorption and migration of the gas in the adjacent seams. Gas drainage from the high-level gas drainage roadway is unsuccessful because it is normally located too high to connect to vertical broken crevices of overlying strata in a timely manner during the initial mining phase, subsequently, the released methane from adjacent seams flows into the goaf. This results in the occurrence of DGC at the longwall face. Thus, based on the analysis of overlying strata movement and the flow characteristics of gas from adjacent seams, we propose an adjacent seams gas drainage technique of a large-diameter blind shaft joined to the high-level gas drainage roadway (LDBS-HGDR) for the initial mining phase. A computational fluid dynamics simulation was conducted to obtain the optimal design of the layout parameters for the LDBS-HGDR at Panel 15201. Compared with the conventional rear high-level gas drainage roadway technique, the gas drainage technique of the LDBS-HGDR is advantageous in terms of ease of construction, low cost, and sufficiently long drainage hours. Moreover, the outflow of gas from adjacent seams can be effectively decreased with this technique, thereby eliminating the hazards originating from the frequent occurrence of DGC.