Abstract: This paper combined numerical simulation and experiment method to conduct steady studies of 3-dimension whole flow passage of a one stage double suction model pump under several working conditions. During the process the RNG k-ε two equation model was selected as the turbulence model, and Eulerian Model as the two-phase flow model, and SIMPLEC algorithm and Zwart-Gerber-Belamri cavitation model were employed to study the internal cavitation characteristics of the double suction pump under different conditions. The main results are as follows: the cavitation processing speeds within the model pump are different under different flow conditions, and under the small flow conditions the speed is slow and cavitation numbers of each stage are relatively smaller, and the bubble development speed is fastest in the large flow condition; When the liquid flow through the impeller hub and the beginning of the double volute clapboard, it produce interactions between liquids or liquids and solids, and change liquid flow angles and cause flow separations and vortexes, and the pressures of these areas decrease because of energy loss, and result in precipitations of cavitation bubbles. This phenomenon is most prominent in 0.6Q_d flow condition, which illustrate that flow angles are easy to change under small flow conditions.