The invention belongs to the technical field of coal gas mining, particularly relates to a low-cost construction process for passing through a goaf and aims to realize passing through a broken and collapsed belt of a permian formation to extract coal gas of carboniferous formation lower group coal. The low-cost construction process includes: adopting a three-holing well body structure for on-ground well drilling, and holing to bed rock for the first time; holing to a position of 22m below a baseplate of the goaf for the second time, lowering a bilayer sleeve, sealing and fixing a bottom port of the bilayer sleeve, and pouring cement between the bilayer sleeve and a well wall for well cementation; holing to a position of 50m below a baseplate of a lower group coal layer for the third time to finish drilling, lowering an inner-layer sleeve, performing well cementation and depth returning to the baseplate of the goaf, and performing well completion after solidification. No sleeve for replacing a drill rod is used, but conventional sleeves are adopted; red soil is filled between the bilayer sleeve and the well wall, cement paste is utilized to intrude the broken belt above the goaf, and the cement is utilized between the bilayer sleeve below the goaf and the well wall for well cementation to stabilize the goaf, so that the circumstance that a drill bit and a wellhole are buried caused by breaking and collapsing of the goaf can be prevented effectively.