An optical image encryption system is designed based on vector decomposition and double random phase encoding. The original image is decomposed into two phase masks. One of the phase masks is encrypted in double random phase encoding system. The other phase mask, which acts as an additional key in the decryption process, is coherently superposed with the decrypted beam to recover the original image. Simulation indicates that the designed encryption system has good encryption and decryption effects, and strong resistance to noise and occlusion attacks. Compared with double random phase encoding system, the designed encryption system has one more key due to vector decomposition, and the key space is thus enlarged. The linearity of the encryption system is broken by vector decomposition. The encryption is resistant to ciphertext-only attack, known-plaintext attack and chosen-plaintext attack, and has high security.