A modified Savonius hydrokinetic rotor was proposed. Flows near the rotor were measured using the time-resolved particle image velocimetry (TR-PIV) technique. Effects of the rotor angle and upstream velocity were described. The rotor geometry was further optimized and the rotor performance was evaluated using the computational fluid dynamics (CFD) technique. The two aspects were dedicated associatively to the illumination of the relationship between flow features and the rotor performance. The results show that the wake of the rotor is featured by large-scale vortices. As the rotor angle and the upstream velocity vary, profiles and even the numbers of the vortices change significantly. Furthermore, the torque coefficient of the rotor depends greatly on the high-pressure zone immediately upstream of the rotor, which is sensitive to the rotor angle. As the upstream velocity increases, the streamwise extension of the vortices is suppressed. The torque coefficient is closely related to the blade shape factor. Long straight edge of the blade is responsible for high maximum power coefficient of the rotor. The high-pressure area near the concave side of the advancing blade extends with increasing blade shape factor. The conclusions shed light on the relationship between flow patterns, rotor geometry and the performance of the drag-type hydrokinetic rotor.