Due to less study on the flow field measurements, especially on the three dimensional flow field measurements, only the calculation results of very limited flow field are validated, therefore the further detail experimental study is necessary. The 3D-PIV particle image velocimetry was applied to measure the flow fields inside pump structure, including a vertical axial-flow pump bell-tube and suction passage. The measurement results under two typical operational conditions of flow rate showed that at the operation condition of design flow rate (Q0), the velocity-field distribution in the impeller inlet section was symmetrical, the axial velocity uniformity of the section was 0.87, and vortex was not occurred, the flow regime in the bell-tube and the impeller inlet was good. The distribution of the velocity field in the impeller inlet-section at large flow rate (1.2Q0) was asymmetric, and the axial velocity uniformity of the section was only 0.70. A pair of strong vortices were formed at each section in both the bell-tube and the space below the bell mouth, which finally entered the impeller and induced vibration. By analyzing the detailed flow structure of vortex core zone, the distribution regularity of circular velocity components in the vortex core was revealed, which were close to zero at the center of the vortex core and increased with the increase of vortex core radius. In the radius range of 3~5 mm, the velocity gradient was the largest and the characteristics of forced-vortex of the vortices were evident. A unit area weighted velocity uniformity and the corresponding calculation formulas of the velocity uniformity in flow section were proposed, and the calculated result by the formulas was more reasonable and practical.