The invention discloses a self-drilling type in-situ measuring device of coal seam gas pressure, is capable of measuring the coal seam gas pressure on the premise of not drawing out a drill rod, and is in particular suitable for measuring the hole easily collapsed coal seam gas pressure. The self-drilling type in-situ measuring device of the coal seam gas pressure comprises a drilling system, a hole sealing system and a pressure measuring system. After drilling reaches a designed drilling depth, a drilling machine is retreated and the drill rod is retracted by a distance which is larger than the width of a packer capsule groove, liquid supply pipelines and drainage pipelines are communicated, a gas pressure measuring lead and an instrument are connected, and cooling water channels are sealed and the periphery of the drill rod and a gap between walls of drilling holes are packed to form a closed space. A gas pressure measurer is opened to measure gas pressure changes in the closed space, and the stable pressure is the in-situ gas pressure. Finally, capsule pistons, packer capsules and casing capsules are operated, therefore, the communication of cooling water channels are restored, a gas pressure probe cavity is sealed, the thread position of a sliding thread casing is restored, external liquid supply pipelines and the drainage pipelines as well as a gas pressure measuring device are dismantled, and the drill rod is pulled out or continuously drilled.