In this paper, a new method of encrypting a color image in one channel based on double-phase encoding is presented. In this method, the colors of a color image are converted from RGB to HSI, so the I component can be used as the original image which could be encoded with double-phase. The kinoform of S is obtained by using double-random phase encryption and the H component can be used as the two phases. It is difficult to recover the S component if the phase keys are not available, because double-random phase encoding is robust to blind retrieval trials, so the security of the method proposed is guaranteed. Computer simulations are presented to illustrate the efficiency of this method.