Disclosed is a sealing method of a high sealing quasi-horizontal drilling hole, which is particularly applied to the operation of mechanical grunting and sealing of a quasi-horizontal drilling hole in underground coal mines. The sealing method of the invention adopts a way of two-stage grunting to seal a quasi-horizontal drilling hole. For a gas drainage hole, the steps are: first plugging the front end and the rear end of a gas drainage pipe by using a bagged double composite sealing material, then starting a grunting pump and pumping seriflux of the sealing material into the gas drainage hole through a first time grunting pipe; waiting until the seriflux grunted at the first time is solidified and an interspace emerges on the upper portion of the gas drainage hole because of the self contractility of the grunted seriflux; and pumping the seriflux of the sealing material a second time into the interspace on the upper portion of the gas drainage hole and the fractured coal body adjacent to the hole through a second time grunting pipe. The sealing method provided by the invention enables the grunted seriflux to effectively seal the inner space of a drilling hole and the fractured coal body surrounding the hole, realizing the maximization of space and crack sealing and meeting the requirement of high sealing. Besides, the sealing method merits in easy operation and low cost.