Abstract: In this paper，expression and simplification of T-TCM decoding algorithm in Log domain is investigated for AWGN.The generation and utilization of extrinsic information is deeply analyzed during the iterative decoding process，and some simplification for branch metric calculation is presented in detail with the trait of Jacobian logarithm formula.According to the simulation results，discarding the correction function of Jacobian will not degrade the T-TCM performance，and a theoretical explanation is given in this paper，further a new T-TCM decoding scheme with no SNR is proposed，which can lower requirements and simplify implementation in the application with no performance degradation.