Abstract: Baum-Welch algorithm most likely results in underflow in practice. Some literatures such as 1-3 introduce "Scaling" algorithm for solving the problem. In applications, it is found that there are some mistakes in the formulae presented in these literatures. The practical calculation shows that the original algorithm often results in poor convergence or even divergence and result in high error rate in speech recognition. This paper analyses the mistakes in these literatures and bring forward the right formulae. The speech recognition system using the revised algorithm can converge well and has low error rate.