In order to improve the effect of optical image encryption ,this paper designs an optical image encryp-tion method based on variable length key. Based on the chaos equation ,a universal algorithm for homogenization of chaotic sequences is constructed. The algorithm is used to generate homogenized pseudo-random sequences ,which are used as variable-length keys. After being modulated by a spatial light modulator ,these keys are converted into pseudo -random phase masks ( PRPM). Two converted PRPM double random phases are used to encode the initial optical image ,and combined with the deformed fractional Fourier transform ( AFrFT) ,the optical image after phase conver- sion is encoded ,Obtain encrypted optical image and complete optical image encryption. The experimental results show that this method can generate sensitive and unique keys for different optical images ,and realize the encryption of dif- ferent optical images. The encryption effect is significant ,the pixel frequency distribution of the encrypted optical im- age is uniform ,and the decrypted optical image is almost the same as the original optical image. The overall encryption and decryption effect is ideal ,which can effectively guarantee the security of the optical image ,Moreover ,the method in this paper can complete the encryption and decryption of optical images in the shortest time of 3s. The encryption and decryption efficiency is higher and the comprehensive effect is better.