The invention provides a gas drainage method by pressure relief of a coal bed bottom plate through ground drilling and loosening blasting. The method comprises the following steps of drilling a plurality of ground drilling wells to the part 2-3cm below a bottom plate of a coal bed working surface on the covering ground of the working surface along the advancing direction of the working surface; adopting an L-shaped drilling technology, and drilling horizontal drilling holes in the bottom part of the ground drilling wells along the driving direction of the working surface, so as to communicate every two ground drilling wells; filling explosives into the horizontal drilling holes, enabling a detonating fuse to penetrate through the adjacent explosives, filling water into the ground drilling wells, and sealing the holes; detonating the explosives, enabling the explosives to blast and loosen the coal bed, and generating a large amount of cracks, so as to reach the purposes of relieving pressure and increasing permeability; sucking the water out of the ground drilling wells and the horizontal drilling holes, connecting a gas drainage pipe into the corresponding ground drilling well, and draining the gas out of the coal bed. The method has the advantages that the ground drilling wells are utilized, a rock layer at the bottom plate of the working surface is blasted, and the gas in the coal bed is drained; the method has the advantages that the cost is low, the safety is high, the gas drainage procedure is simplified, and the like.