Abstract: Abstract: Changing rules of water use efficiency (WUE) in rice ecosystem can significantly contribute to the water-carbon cycle in paddy fields, thereby to optimize water-carbon management in agricultural production. Taking double-cropping rice of Poyang Lake Plain (belongs to middle and lower reaches of Yangtze River) as the research object, a field experiment was carried out from 2011-2013 and 2018, where the CO2 and H2O flux for 3 years was measured using eddy covariance (EC) method. A series of quality control processing was performed on the raw data of flux using the EddyPro software and the Tovi software, such as the double coordinate rotation, WPL correction, u* threshold detection, energy balance residual correction, and footprint analysis. A marginal distribution sampling (MDS) was used to make the gap filling of missing data, further to obtain a complete dataset. An investigation was made to explore the seasonal variation of ecosystem-scale WUE, as well as the relationship between WUE and its constituent elements net ecosystem productivity (NEP) and evapotranspiration (ET). An attempt was also made to compare different WUE in the rice fields at different latitudes, including early rice, late rice and other farmland ecosystems. The results showed that in the seasonal scale variation curve of rice WUE, there was a tendency of increasing first and decreasing later, following with the variation characteristics of NEP. There was a significant positive correlation between rice WUE and NEP, but not with ET in most rice seasons, whereas, the effect of late rice NEP on WUE was higher than that of early rice. This indicated that the seasonal variation of WUE was dominated by NEP, which depended mainly on the growing development of rice on the scale of growth period. The seasonal peak of rice WUE appeared in the jointing-booting stage, and the WUE of middle stage of growth (from jointing-booting stage to heading-flowering stage) was maintained at a high level, with a mean WUE of 5.8 g/kg for early rice and 5.5 g/kg for late rice. In early rice, the WUE of late stage of growth (from grain-filling stage to yellow-ripening stage) was higher than that of early stage of growth (from the stage of seedling establishment to tillering), while that of late rice was the opposite. Both stage of seedling establishment of early rice and yellow-ripening stage of late rice showed an obvious negative WUE, where the paddy field was carbon source at the moment. The mean WUE of the whole growth period of early rice was (3.3±0.3) g/kg, slightly lower than (3.4±0.4) g/kg for late rice. The reason was that the NEP of late rice was stronger, while, the WUE of early growth period for early rice was lower. The synchronism between all-day WUE and daytime WUE was high, where the mean daytime WUE of early and late rice were 5.5 and 6.1 g/kg, respectively, indicating that the effect of nocturnal respiration consumption in rice ecosystem reduced the WUE by more than 40% as a whole. The rice WUE in the middle and lower reaches of the Yangtze River was higher than that in the tropical areas, such as the Philippines and Brazil, and lower than that in the Northeast China. With the increase of geographical latitude, the WUE of paddy field increases significantly. The rice WUE was also significantly lower, compared with other farmland ecosystem, such as wheat and corn. There were interannual differences in WUE of growth periods for early rice and late rice, where the relative variation was 9.2%-12.4%, due to the interannual variation of NEP and ET caused by the change of environmental variables, such as temperature, and radiation. The interannual variation in WUE of heading-flowering stage was the maximum, with the amplitude of 1.9-2.3 g/kg. The findings can provide a sound basis to evaluate agricultural water use efficiency, and thereby optimize water-carbon management models in paddy fields in the middle and lower reaches of the Yangtze River in China.