The invention relates to a construction method for opening off a small-section underground chamber of a coal mine rock roadway into a large-section underground chamber. The construction method comprises the following steps of: drilling and blasting on the top of the rock roadway at a design starting position of the underground chamber at an angle of 70 degrees tilting relative to the longitudinal direction; layering and roof-ripping; clearing by using a scraper loader; brushing the wall while spraying slurry; and the like. The construction method has the advantages of optimizing construction processes, greatly increasing the production efficiency, reducing the labor intensity of workers and increasing the safety coefficient during construction. The construction has high adaptability; the small-section underground chamber can be opened off into the large-section underground chamber on the basis of the conventional small-section rock roadway; the time is saved; and the resource waste is also avoided.