Aiming to the problems caused by the settled limit Sign Chance Ratio(SCR) stopping criterion，by using the convergence characteristic of turbo code，an alterable limit SCR stopping criterion is proposed based on the Signal to Noise Ratio(SNR) estimation. This criterion chooses different limits based on different SNRs，and it is improved especially at the low and high SNR situation. The improved SCR criterion could reduce the average number of iterations efficiently as well as assure the bit error rate performance at the low and high SNR situation. It accelerates the decoding process and improves the decoding efficiency. The performance is improved significantly especially at a lower SNR condition.