To study the characteristics of rainfall-runoff generation processes in the plain area of the Taihu Lake Basin, the Jintan hydrological experimental area was selected to carry out the observation of rainfall-runoff processes.The specific elements that affected the runoff generation during rainfall events were extracted and the whole hydrologic processes of two typical runoff generation events, which were dominated by saturation-excess and infiltration-excess mechanism respectively, were analyzed. The results indicate that the experimental area is dominated by the mechanism of saturation-excess and the phenomenon of infiltration-excess is rare. The runoff generation processes in the plain experimental area under the condition of long-duration light rainfall can be divided into four stages: soil water filling, surface depression filling & ditch outflow, whole area outflow, and recession after rainfall. The runoff coefficient in the ditch outflow stage is small and the outflow is mainly composed of the rainwater falling on the area of the ditch. The whole area outflow stage belongs to the infiltration-excess mechanism under the condition of soil moisture reservoir is saturated and the rainfall intensity threshold is about 0.1mm/min with a large runoff coefficient. The first two runoff-generation stages may occur simultaneously in the short-duration heavy rainfall. If the soil is not saturated at the whole area outflow stage, it is a traditional infiltration-excess dominated runoff event with a large proportion of infiltration. While if the soil is saturated, it belongs to the infiltration-excess mechanism under the condition of soil moisture reservoir is saturated.