The sequences of Alternating step Generator are analyzed through the method of information theory and obtain some significant results. For example, the mutual information between clock-controlled sequence and clock-controlled output sequence is zero, i.e. one can’t get any information about clock-controlled sequence only from output sequence. Further, the lower bound of mutual information between clock-controlled input and output sequences is given. The results obtained can provide theoretic reference for designer and analyst who study clock-controlled generator.