The invention relates to a method for layering, stage multi-level blasting, hole expanding and crack expanding of an oil-gas-bearing rock. A drilling machine drills a vertical hole to the oil-gas-bearing rock, multilayer horizontal build-up is conducted in the oil-gas-bearing rock, and a casing pipe is contacted and fixed with an outer shale body and extends into a curve section; a light pipe is contacted and fixed with the outer shale body in a horizontal section, a packer is used for sealing the staggered position of the light pipe and the casing pipe, the light pipe with the preset length is arranged inside the horizontal section, and except the light pipe section, the residual horizontal section is an uncased hole; and a hydraulic expansion device is used for sending explosives to different positions of the horizontal uncased hole section, the explosive quantities increase gradually at the same position, stage blasting hole expanding and high-pressure gas explosion cracking at the position of the uncased hole are completed, and backwashing is conducted for different-stage blasting broken rocks to clear the uncased hole. Step-by-step blasting is conducted in a direction from the remote end of the uncased hole to the light pipe during blasting cracking, different levels of explosive quantities are adopted, the quantity at the remote end of the uncased hole is the most and the quantities are reduced gradually towards the curve section, so that the light pipe is protected, the backwashing of blasting broken rocks is facilitated, and big hole collapse and cracking are achieved.