Abstract: Abstract: To investigate the flow field distribution at the regions of the impeller and guide vane inlet and outlet under condition of 0.8Qopt, the axial flow pump with ns=700 was selected for model scaling and structural modification, from which we could get a test bed suitable for a 2D-PIV internal flow field test. Structural modifications included: the conventional metal material was replaced with transparent organic glass material to achieve the purpose of internal visualization; the conventional conical diffuser vane was designed into cylindrical shape to reduce the complexity of optical refraction; the runner chamber was merged with a guide vane casing to form as a whole to eliminate the occlusion of the flange to an internal flow field between the region of impeller and diffuser vane; the bearing within the diffuser vane was moved backward and rib plates were installed to make the load transfer to foundation smoothly. Based on the above methods, an experiment pump section was modified successfully, and the efficiency of the experimental pump reached 73.79%, which was close to that of the prototype pump. It was indicated that less damage occurred to the original flow field with structural modification. During the PIV measurement, the shaft encoder and synchronizer were used for better synchronous effect. Meanwhile, an ideal experimental result was obtained by using organic glass hollow spheres as tracer particles, and a new calibration method. As shown in the original PIV images, the particle distribution was homogeneous with most particles appearing to be micro-exposure, which meant an ideal experiment effect. From the analysis of the experimental result, it was indicated that, owing to the effect of the tip leakage flow at the 0.8Qopt operating point, the inflow at the rim of an impeller leading edge deflects to the hub side, but the whole flow field is evenly distributed on the front section of an impeller; a clockwise vortex with an outer diameter larger than the hub between the hubs of an impeller and the guide vane causes a great impact on the flow field near the root of the impeller's trailing edge, and the flow field in the axial clearance of the impeller trailing edge and the guide vane leading edge tends to deflect to the outer rim integrally; the streamline direction after the guide vane outlet shifts towards the hub side, which leads to a local high-speed zone at the rim of trailing edge. Moveover, the velocity of the high-speed zone is about double as much as the average velocity of the flow field downstream from the guide vane.