Recently，a large number of chaotic cryptosystems have been proposed，yet many of them have the drawbacks of lack of robustness and security. In this paper，we point out the weakness of a very recent block cipher algorithm which is based on the chaotic map and give the improved scheme of it. We provide the chosen plaintext attack to recover the permuted plaintext string. It is shown that the generation mechanism of binary sequences which depends on the key but not on the plaintext facilitates leakage of information and is vulnerable to attacks. Based on such a fact，we give the improved scheme to achieve higher security.