In order to reduce the harm of coal mine gas and realize comprehensive utilization, the large diameter gas hole project is encouraged and supported in China. This paper shows a large diameter gas hole with diameter of 850mm and termination depth of 401.6m and in detail introduces the special construction technologies after small diameter hole drilling through the tunnel and related technical measures in drilling methods, drilling tool grading, drilling technical parameters, flushing fluid, borehole rolling and straightening, protection casing installation and consolidation at hole bottom, circulating mud filling, pipe lowering and fixing and hole construction for drainage.