The riverine transport of terrestrial particulate organic carbon (POC) is the major link between terrestrial and marine systems, which affects the global carbon cycle. It is predicted that extreme rain events will increase in the future, therefore, understanding the responses of riverine export of terrestrial POC, especially factors controlling the response of river systems to extreme rain event, is essential for projecting the future carbon cycle. Here we quantified the POC sources in a mid-scale river during base flow and extreme rain event by combing bulk properties (elemental composition and stable carbon isotope) and lignin phenols. We found that riverine suspended matter mainly comes from aquatic organism during base flow. During flood, POC in river is mainly from surface soil and C3 plants. The source of riverine suspended matter significantly changes during the transition from the base flow period to the flood period. However, it remains stable with continuous increase in water discharge during the flood period. We further estimated that during the ten - days event the export of POC and particulate lignin was 21% and 31% of their annual flux, respectively. During a period of time (approximately 20 days) after flood, the POC is still dominated by soil sources, and due to the hysteresis effect of flushing, peaks of soil sources occur only after the discharge peak.