The invention discloses a bed-parallel drilling slot pressure relief permeability increase method which includes the steps: (1) performing construction on bed-parallel gas extraction drilling by the aid of a high-pressure hydraulic slotting device to form a drilled hole according to bed-parallel gas extraction drilling design requirements; (2) leading high-pressure water into the high-pressure hydraulic slotting device without withdrawing a drill to rotatably and circumferentially cut coal to form a disc-shaped slot; (3) withdrawing the drill after the disc-shaped slot is formed, and radiallycutting the coal without rotating a drill bit and a drill rod to form a radial slot; (4) repeating the step (2) and the step (3), and forming a plurality of disc-shaped slots and a plurality of radialslots in the same drilled hole; (5) repeating the step (1) to the step (4), and performing drilling and slotting on the other bed-parallel drilled holes. According to the method, horizontal and vertical stress of a coal layer is reduced, drilling gas extraction concentration and flow is improved, the bed-parallel drilling number of a working surface is decreased, and the method is simple and convenient to operate.