In this paper,informatioin loss of Fourier-transform-based hidden images during JPEG compression is studied from the viewpoint of image energy. The method of computing energy loss for a given compress rate of the hidden image,and the empirical formula describing the relations between the compress rate and the rate of energy loss of the hidden image are presented. Experiment results show sthat the loss of image energy caused by the image compression can result in aberration, nosie,and depressed resolution.