The invention discloses a brake method of an expansion machine. The method is characterized in that air produced by an air separation unit is used as a brake gas of the expansion machine. The method of the invention avoids the influence of atmospheric environment on the brake operation of the expansion machine so that the expansion machine can continuously run all year around without influencing the refrigerating effect, the operation state of the expansion machine is more reliable to achieve the maintenance-free aim, and the clean gas after brake can be still input into a purifying machine to be continuously utilized. The heat generated in the brake process can be reutilized, electricity consumed by the purifying machine is reduced, and the triple aims of saving energy, saving air filters and freeing maintenance are achieved.