In order to research the internal flow mechanism of screw centrifugal pump, the PIV testing technology was used after the pump was improved with the transparent handling. The velocities of shaft section and radial section at different stages were obtained. The flow of each section was received and then the internal flow in screw centrifugal pump was revealed. The results showed that the perturbation of physical parameters at the whole shaft section was bigger than the one at radial section. The vortex was appeared obviously at shaft section of impeller passage and the vortex volume increased significantly in the impeller passage which was deep into volute. With the depth of the fluid flow, it caused fluid force reallocation by the spiral vane segment. At radial section, velocity was rotated along with the direction of the flow line. Simultaneously, there was a trend that fluid moved from the center to the outside due to the centrifugal force from the centrifugal section, which played an important role after the spiral section. The spiral propulsion of spiral section and the energy conversion of centrifugal section were cooperated with each other to make up the working process of screw centrifugal pump.