In this paper, the influence of the El Niño-Southern Oscillation (ENSO) cycle on the sensitivity of nonlinear factors in the numerical simulation is investigated by conducting numerical experiments in a simple air-sea coupled model for ENSO prediction. Two sets of experiments are conducted in which zonal nonlinear factors, meridional nonlinear factors, or both are incorporated into the governing equations for the atmosphere or ocean. The results suggest that the ENSO cycle is very sensitive to the nonlinear factor of the governing equation for the atmosphere or ocean. Thus, incorporating nonlinearity into air-sea coupled models is of exclusive importance for improving ENSO simulation.