First numeric computation of fluid field inside jet pump is conducted with continuity equation and momentum equation of steady fluid under right-angled two-dimension coordinates, and the random walk model is given. As for the method of integrating coupling pressure and momentum equation is adopted. The result is conforming to the flow of reality. Furthermore, the relation of random walk times and computing time and computing errors is analyzed in-depth. The relation of iterative times and computing errors is analyzed. It establishes a foundation for studying the flow field of jet pumps to use the Monte Carlo Method. At the meantime it develops a new field.