Abstract: The security of a cryptosystem proposed in the article of Chaotic image scrambling algorithm based on sort transformation was analyzed. The whole key space of the image scrambling cryptosystem is too small to prevent the ciphertext-only attack when realizing in finite computing precision. The address codes of transposition can reappear by chosen-plaintext attack or chosen-ciphertext attack. A key recovery algorithm was presented by reverse iterating the Logistic chaotic map. The theoretical and experimental results verify that the key recovery algorithm is feasible and efficient. Two primary reasons for the unsafe problem are the restriction of finite computing precision and the weakness of linear transformation. Five possible improved methods and their effects were also discussed.