In order to study the flow mixing characteristics of lower plenum of small pressurized-water reactor (PWR), this study conducts a hydraulic experiment on the reactor model in scale of 1∶3 based on the scaling method, and obtains the flow mixing factor matrices for the core inlet under the balanced and unbalanced flow conditions of the cold leg by measuring the solution concentration change. The results show that under the balanced flow condition, the changes of cold leg flow have slight effect on the core inlet flow mixing factor matrix, while under the unbalanced flow condition, the fuel assembly flow mixing factor near the outlet pipe is significantly affected by the flow unbalance, and that small change occurs to the flow mixing factor in the central region. Therefore, the flow mixing characteristics of lower plenum of small PWR are stable at the balanced flow, and focus should be put on the change in flow mixing characteristics of fuel assemblies near the outlet under the unbalanced flow condition.