In order to avoid the characters of current double-random phase-encoded optical image encryption, whose key and ciphertext are large in size, and has weak ability to resist the attack of selecting plaintext and ciphertext, a lossless compression optical image encryption method was proposed based on Logistic-Logistic cascade chaos and vector decomposition. Firstly, the plaintext is divided into two images by space sampling and scrambling. Then the images are synthesized together by the interference. At last, the ciphertext can be gotten by putting the image into a double random phase encoding system. The scrambling improves the security of the encryption system, because it can avoid plaintext information to be seen after interference. The interference reduces the volume of the ciphertext to about the half in size, which is convenient for ciphertext transmission. The decryption method based on unit-mode vector decomposition could avoid the problem of low resolution of the decrypted image caused by the existing compression method. Logistic-Logistic cascade chaos can reduce the volume of double random phase encoding, improve the randomness of the sequence and keep the fastness of the Logistic chaos. It can also solve the problems of uneven distribution of the Logistic sequence. The HASH value SHA256 of plaintext is associated with the secret key strongly, which makes the whole system achieve the encryption effect of one image with one key. This also improves the avalanche effect between plaintext and ciphertext, and enhances the capability of resisting the attack of selecting plaintext and selecting ciphertext.