A decoding method applied to a convolutionally coded signal is provided. The method includes: adjusting first input information according to a first scaling factor to generate first a-priori information; b) decoding the convolutionally coded signal according to systematic information and the first a-priori information to generate first extrinsic information; c) adjusting second input information according to a second scaling factor to generate second a-priori information, wherein the second scaling factor is generated according to the first extrinsic information and the first a-priori information; and d) decoding the convolutionally coded signal according to the systematic information and the second a-priori information to generate second extrinsic information. One of step (b) and step (d) further generates a-posteriori information as a decoding result.