In order to research the hydraulic stability of axial flow pump in the double hump region under small flow conditions, the relationship between the head-flow curve of the axial flow pump and the characteristics of the internal flow field was discussed, and a test and research on an axial flow pump with a specific speed of 825 was carried out. The internal and external hydraulic characteristics of the impeller under small flow conditions were obtained through the characteristic energy curve test, the internal flow field tracer particle trajectory was captured by a high-speed camera, the wall pressure pulsation collection, and the impeller inlet and outlet section velocity measurement by LDV equipment. According to the results of energy characteristics, it was found that there was a double hump phenomenon in the head-flow curve of the axial flow pump. The first hump appeared between 0.5QBEP and 0.6QBEP, corresponding to the tangential fluctuation of the speed on the side of the inlet flange, the rapid increase of the velocity circulation, and the extreme peak-to-peak pressure pulsation. The test results showed that the first hump was related to the generation of the impeller inlet reflux. The second hump occurred between 0.33QBEP and 0.4QBEP, the test results showed that the second hump was related to the drastic changes in the flow field of the impeller outlet. The impeller inlet reflux was generated during the second hump formation, although the strength was increasing and the influence range was wider, the pressure pulsation peak-to-peak value on the wall of the inlet side of the impeller chamber was no longer increased. The pressure pulsation peak-to-peak value on the wall of the outlet side of the impeller chamber was increased rapidly, and the unevenness of the velocity distribution between the outlet hub and the blades was obviously deteriorated, the reflux area of impeller outlet was increased, which indicated that the second hump was related to the drastic changes in the flow field of the impeller outlet.