A novel all optical analog-to-digital conversion system based on fiber nonlinearity was brought forward by Sho-ichiro. In this paper, we analyzed the theory of the system and simulated the process of sampling and quantizing. The split-step Fourier method was used for the numerical simulation. The result of numerical simulation revealed the influence on the sampling and quantizing which was caused by chromatic dispersion and nonlinear effects (XPM, SPM and FWM) in the fiber. Based on the result, the proper parameters of the system were selected.