The invention discloses an extraction method for coal bed gas in a composite old mined-out area formed through a lower tool post method. The method comprises the steps that a horizontal well is constructed at a middle rock layer between an upper old mined-out area and a lower old mined-out area, extraction is carried out, and the coal bed gas extraction amount within the unit time is recorded as Q1; when the extraction amount is decreased to be below 10-20% of Q1, fracturing is carried out for extraction again, and the coil bed gas extraction amount is recorded as Q2; when the extraction amount is decreased to be below 10-20% of Q2, fracturing is carried out again, and the extraction amount is recorded as Q3; when the extraction amount is decreased again to be 10-20% of Q3, carbon dioxide displacement is carried out; then, extraction is carried out again and not stopped until the extraction amount is decreased to 6 m<3>/min. By means of the method, the problems that only a ground vertical well drilling method is adopted for extracting the old mined-out area in the prior art, the effect contact area of an extraction drilling well and an old mined-out area storage layer is small, and the extraction range is limited are solved; sources near the upper old mined-out area coal bed layer are extracted together with lower old mined-out coal bed gas, and the cost is saved.