The internal flow field in a vertical axial flow pumping system was simulated by CFX software using the Reynolds averaged N-S equations and RNG k-ε turbulence model. Focusing on details of the inlet flow field, the flow characteristics in the inlet and outlet passages of pumping system were analyzed based on the calculation of different operating conditions with the flow rates range 180~340L/s at the rated rotating speed. The results show that rotating of impeller has effects on the distribution of axial and tangential velocity components at the pump inlet, but the numerical values are relatively small. Outlet circulation of guide vane has great influence on flow pattern in the outlet passage. The flow distribution for both sides of the dividing pier is not symmetric, especially for small flow rate condition, the helical flow occurs at the right side of dividing pier. A good agreement is achieved in the performances of the pumping system between the predicted data and experimental data at the best efficiency point, which can meet the requirement of practical application. 