As one of the candidates of cellular phone TV standard, the DAB-based CDMB adopts convolutional codes as its channel coding. In this article, we present a new sum-product decoding algorithm, where parity-check matrix is constructed under the circumstance that the sending encoder is unaltered or partially modified. It shows that our method obtain about 1dB gain higher than the traditional Viterbi algorithm in high SNR regime and with the same quantified bits. It is of importance in performance enhancement for the convolutional codes based system in practice.