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A method to fulfil the digital image encrypting by the computer-generated hologram of an optical system is present. The optical system is composed of a fractional Fourier transformation. To make the hologram, a vertical plane wave is used as a reference beam. In the encrypting process two random phase masks are needed. First a digital image mixed with a random phase mask undergoes a fractional Fourier transformation. Then we add another random phase mask to the latter plane of the fractional Fourier transformation and the latter plane passes a Fresnel diffraction process. The Fresnel diffraction field and the reference light field interfere to form a hologram. The two random phase variables, the distance of Fresnel diffraction and the parameters of the fractional Fourier transformation can be regarded as keys for the encrypting of a digital image. In the decoding process a vertical plane wave is used as a reconstruction light. Computer simulation shows the feasibility of our encrypting method for a digital image. Because the key number of the encrypting method is more than that in the single encrypting process such as Fresnel diffraction encrypting process, the safety of the encrypting is greatly enhanced.