The invention discloses an underground gasification method. The underground gasification method comprises the following steps: filling a whole or a preset part of a primarily gasified worked-out section of an underground gasification furnace; perforating and gasifying coal pillars at the periphery of the filled worked-out section; filling a primarily mined worked-out section of the original underground gasification furnace, and performing secondary recovery on residual coal pillars. The underground gasification method has the following advantages that the method is simple, the cost is low, and an intake unit, an outlet unit and the like of the original underground gasification furnace can be used; the application range is wide, and the technical problem that residual coal resources cannot be recovered after the underground gasification furnace is subjected to primary gasification mining is effectively solved.