The invention belongs to the technical field of coal mine safety, and particularly relates to a coal seam air injection expansion secondary permeability improvement method adopted after liquid CO2 blasting and used for promoting gas extraction. According to the method, blastholes and gas extraction holes are formed in a work face at the same time and formed at equal intervals, and liquid CO2 blasters are mounted in the blastholes to conduct liquid CO2 blasting; then, gas extraction pipes are laid in the gas extraction holes, the gas extraction holes are sealed, air of a coal mine down-hole compressed air system serves as an air source of air injection, coal seam air injection expansion secondary permeability improvement is conducted, the air injection air source is connected into the blastholes, the blastholes are sealed, an air injection and gas extraction system is started, and gas extraction is conducted. By the adoption of the method, gas extraction scalar quantity is greatly increased, and huge economical and social benefits are achieved; meanwhile, the technical process is simple, and the method has very good application and popularization prospects.