Aiming at the problem that the numerical attack algorithms such as hybrid input-output (HIO) algorithm attack the JTC encryption system with long running time and low attack efficiency, an optical ciphertext-only attack system based on optical path iteration is designed. The system utilizes the characteristics of high-speed parallel processing of optical system, and replaces the digital Fourier transform by optical Fourier transform, which significantly reduces the running time of the algorithm and effectively improves the attack efficiency. The simulation results show that compared with the HIO algorithm, the optical ciphertext-only attack system effectively reduces the running time of the algorithm and significantly improves the attack efficiency of the JTC encryption system under the same number of iterations.