The double random phase real-value encryption and decryption method is proposed for the optical image security system.A phase-only image to be encrypted together with a random phase mask in space plane is Fourier transformed .Then it attached anther random phase mask in Fourier plane is enlarged and reverse-Fourier transformed .Real part of the result is used as an encrypted image. The decryption of the real-value encrypted image can be carried out in the same means with encryption. That gives the exact original image. The method of encryption and decryption is simply performed and encrypted image is real-valued,so it is convenient for output.