An audio signal processing method is disclosed. The audio signal processing method includes receiving a residual and long term prediction information, performing inverse frequency mapping with respect to the residual to generate a synthesized residual, and performing long term synthesis based on the synthesized residual and the long term prediction information to generate a synthesized audio signal of a current frame, wherein the long term prediction information comprises a final prediction gain and a final pitch lag, the final pitch lag has a range starting with 0, and the long term synthesis is performed based on a synthesized audio signal of a frame comprising a preceding frame.