We use the iterative split-step Fourier method to simulate the eight quadrature amplitude modulation all-optical orthogonal frequency division multiplexing (OFDM) system employing different constellation design schemes. It is found that properly narrowing down the modulus difference between constellation points has an impact on the whole system’s ability to resist nonlinear effects. The results show that by means of changing the modulus values of the constellation points, the system bit error rate will be significantly reduced when the optical signal-to-noise ratio is high. Furthermore, by comparing four different constellation diagrams, we propose an alternative solution to reduce the distortion brought by the nonlinear effects on all-optical OFDM systems, that is, the constellation points can be appropriately redesigned to improve the performance of high nonlinear effect systems.