The invention discloses a decryption method of an image encryption algorithm based on a chaotic S box. The method comprises the steps of selecting 256 grayscale images with pixels of all 0, all 1, all2, ... ,all 255, and encrypting the 256 grayscale images through an original encryption algorithm, so as to obtain 256 plaintext pixel matrixes; then acquiring a pixel matrix of an encrypted ciphertext image; traversing and comparing the pixel of each position in the 256 plaintext pixel matrixes with the pixel of corresponding position in the pixel matrix of the ciphertext image, and finding outeach original pixel corresponding to the pixel in each position of the ciphertext image, so as to obtain an original image corresponding to the encrypted ciphertext image. Through adoption of the method, the encrypted image can be decrypted successfully without acquiring a key, the image recovery rate is high, and the calculation process is simplified. The method is suitable for an original encryption algorithm and any S box-based encryption system, and has the advantages of high image recovery rate and wide application range.