Block modulation-scrambling image encryption is one of the common encryption methods for reversible data hiding in encrypted image(RDH-EI). It can effectively improve the embedding capacity of the algorithm and resist the existing ciphertext only and known plaintext attacks. For block modulation-scrambling image encryption, a key stream estimation method under known plaintext attack is proposed in this paper. First of all, the definition of image difference block is given, and it is pointed out that the ciphertext block generated by block modulation keeps the difference block unchanged with high probability. On this basis, a fast block scrambling key estimation method based on pseudo difference image construction and difference cube mean index search is proposed. The relationship between the cube mean distribution of the difference block and the block size and the accuracy of the scrambling key estimation is discussed. Finally, the possible solutions to improve the security of image encryption are given. The texture complexity and block size of the plaintext image are the main factors that affect the block scrambling key estimation accuracy and algorithm time complexity. When the block size is larger than 3×3, the accuracy of all test image block scrambling secret key estimation is more than 70%, at this time, the content information of ciphertext image is seriously leaked.