A long-term observation (2007-2012) of CO2 fluxes during the rice and broad bean growing seasons was conducted in the Erhai Lake riparian farmland ecosystem using eddy covariance techniques. The study clarified the characteristics of CO2 flux changes and the source/sink function of the farmland ecosystem under the condition of crop rotation of the two crops. The results showed that: (1) The climatic conditions of the lakeside farmland in Erhai Lake are quite different when rice and broad bean are different underlying surfaces. In the rice growing season, the temperature (Ta) and relative humidity (RH) were relatively high, the saturated water vapor pressure difference (VPD) showed a trend of first decreasing and then stabilizing, the total solar radiation (Rg) fluctuated significantly, and the wind speed (WS) and friction velocity (ustar) showed a weak downward trend. In the growing season of broad bean, Ta showed a trend of first decreasing and then increasing, RH showed a gradual decrease, while VPD and Rg showed an upward trend, and WS and ustar showed a trend of first increasing and then decreasing. (2) The CO2 fluxes in the Erhai Lake riparian farmland ecosystem showed obvious diurnal and seasonal variation characteristics, showing a "U"-shaped diurnal variation pattern and a single-peak double-trough "W"-shaped seasonal variation pattern. During the rice growing season, the peak of CO2 fluxes occurred during the jointing and heading stages; during the broad bean growing season, the peak of CO2 fluxes occurred from podding to seed filling. (3) On the daily scale, the main controlling factor for the change of CO2 fluxes in the Erhai Lake riparian farmland ecosystem was RH, while on the hourly scale, it was Rg. (4) The rice-broad bean farmland ecosystem generally showed carbon sink characteristics. The net carbon exchange during the rice growing season was 503.8 gC.m^-2, and the gross primary productivity was 866.6 gC.m^-2; the net carbon exchange during the broad bean growing season was 143.5 gC.m^-2, and the gross primary productivity was 716.2 gC.m^-2. The carbon sequestration capacity of the rice growing season was about 3.5 times that of the broad bean growing season.