Atmospheric wet deposition is an important source of carbon and nitrogen in watershed ecosystems and has a substantial effect on health and stability of ecosystems. In the typical subtropical Xiangxi watershed, seasonal dynamic characteristics of C and N in wet deposition and rainfall-runoff export events were monitored and analyzed to discuss C and N coupling processes and matter equilibrium relationships between deposition and rainfall-runoff export. The results showed that, in the process of rainfall and rainfall-runoff in the Xiangxi watershed, the monthly average concentrations of C and N had obvious seasonal differences. The C and N concentrations in atmospheric wet deposition were higher in the dry season than that in the wet season, and the values in rainfall-runoff were greatly affected by local agricultural activities, such as fertilization. The average concentrations of DOC and TN in atmospheric rainfall were 5.28 mg/L and 5.91 mg/L, respectively, and the concentrations of C and N in rainfall-runoff were significantly lower than that in rainwater. Between DOC and TN concentrations, there were very significant positive correlations in rainwater, no significant correlations in rainfall-runoff, and significant negative correlations in conventional water. Nutrient sources in each process were the main factors affecting the relationship between C and N. Because of the erosion to soil by heavy rainfall, C:N in rainfall-runoff was obviously higher than that in conventional water. The ratio in rainwater was similar to that of conventional water, indicating that atmospheric wet deposition was the main source of C and N in the watershed. The main reason was that most of the rainfall events were small and medium rainfall in the Xiangxi watershed, and the annual rainfall had a limited effect on soil erosion. Therefore, C:N in conventional water was almost unaffected by soil. The mean value of C:N in conventional water in the Xiangxi watershed was 2.81, which was much lower than the suitable value (approximately 6.6) for plankton growth according to the Redfield ratio, indicating that exogenous N input had caused the water environment to be in a state of excess N in this watershed, and long-term output would increase the degree of nutrition of the downstream Poyang Lake. Rainfall processes had a small effect on C input-output balance but had a greater impact on N input-output balance in the Xiangxi watershed. The annual wet deposition fluxes of DOC and TN were 69.41 kg hm^-2 a^-1 and 77.23 kg hm^-2 a^-1, respectively. The level of C and N deposition was controlled by regional rainfall and air pollution. Deposition flux of TN in the Xiangxi watershed ecosystem accounted for 33.13% of the annual average nitrogen fertilizer application in this region. Therefore, the large number of nutrients input into subtropical watershed ecosystems by atmospheric rainfall cannot be neglected.