A new method for color image single-channel encryption is proposed. According to tricolor grating theory, a color image is encoded into a gray scale image, and then the gray scale image is encrypted by double-phase encoding. Thus by combining multi-channel with single-channel, only one laser is needed for encryption. With this method, both single-channel encryption and high security are ensured, and the system is simpler and more practical. Computer simulation proves the validity of the proposed method.