The invention discloses a depressurized extraction method for cutting through a roof or a floor of a coal seam by an abrasive water jet, which comprises the following steps: a. drilling a bedding drill hole to set depth on the roof or the floor of a coal-rock mass or a hard layer by using a drilling machine through a drill stem and a slotting drill bit; b. delivering a high-pressure abrasive water jet of which the pressure is 25-30MPa to the slotting drill bit through the drill stem, wherein the drill stem rotates in situ; forming a radial slit on the hole wall of the bedding drill hole, wherein the slit penetrates the roof or the floor of the coal-rock mass or the hard layer and a soft layer; c. enabling the drill stem to rapidly retreat for 1.5-3m so as to form a new slit; d. repeating the step c until a plurality of radial slits are formed on the bedding drill hole, and moreover, reserving a safety rock pillar of at least 5m at the slit located at the position of the orifice of the bedding drill hole at a certain distance from the orifice; e. retreating the drill stem and the slotting drill bit and sealing the hole; and f. extracting gas by a gas extraction system. The depressurized extraction method for cutting through the roof or the floor of the coal seam by the abrasive water jet has the beneficial effects that the drill hole does not have potential safety hazards and is easy to form; a hole sealing effect is good; the time of reaching the standards of extraction is short; the production efficiency is high; and the extraction is safe.