A new automatic measurement system of flow was applied. Simulated rainfall experiments were conducted under constant and variable rainfall intensity in small watershed in the laboratory. Runoff processes of the watershed branches were monitored realtimely. Under the constant rainfall intensity, 12 rainfall experiments were conducted involving a complete combination of four kinds of rainfall intensity (25, 50, 75, 120 mm/h) and three kinds of rainfall duration (5, 10, 15 min). The experiments of variable rainfall intensity were conducted at 30-70 mm/h and 70-30-70 mm/h rainfall events. Under the constant rainfall intensity, the maximum averaged relative error of the measured values of stable flow between the automatic system measurement and the manual measurement is only 2.02%. The runoff diagrams had three obvious stages of initial runoff, stable flow and recession flow. The severe degrees of fluctuation in the processes of initial runoff and recession flow and the flow of stable flow were increased with the increasing of rainfall intensity while the duration of the stable crest segment was directly proportion to rainfall duration. The runoff processes at the watershed outlets delayed to those at the branches, but the processes no matter at the outlets or branches of the watershed were identical in shape. Under the variable rainfall intensity, the runoff processes had no stable segment and the flow changed with the rainfall intensity. The results are significantly meaningful for understanding the runoff process. The results also can provide reliable experimental support for related model research.