The computation of Green's function in spatial domain is the key difficulty in solving multilayer structure. According to the nonuniqueness of inverse problem, a novel method based on discrete complex image method(DCIM)—fixed real image method(FRIM) is put forward.According to this method, we use fixed images at real instead of complex locations to approximate the spectral Green's function, the images real locations are selected according to the classic image theory, and the complex amplitudes can be obtained using simple point match method. This method is simpler and faster than the complex image method for avoiding the fitting procedure of Prony or GPOF. A group of computation results are given and validated by DCIM.