Baum-Welch algorithm, which is often troubled with overflow, is one of the basic methods in the field of speech signal processing. People have to adjust the inner parameters constantly. So in this paper, a modified Baum-Welch algorithm is presented to avoid the overflow completely. With this algorithm manual adjustment is not needed and the computation accuracy is guaranteed. There is no significant extra cost of computation and storage. The feasibility of the new algorithm is shown in the experiment.