Water-level fluctuations are very important to the formation and distribution of the floodplain grassland around Lake Poyang, and different factors of water-level fluctuations (aspects of water regime) have different effects on wetland plant species. In this study, we used CART model to analyze the relationship between the distribution of Carex and five water-level fluctuations factors (annual water level range,R;annual frequency,F;annual mean water level,M;date of emergency after the longest inundation,D;annual total dewatered time,T), with plant data from sampling around Poyang Lake and water regime date offered by local hydrology bureau. The results showed that:1) the main influence factors of the distribution of Carex were T and R;2) the optimum condition to Carex community should be described as T ≥ 4.619 and R ≥ 10.41,which means annual total dewatered time more than 84 days and annual water level range larger than 11.3 meters. Further discussion indicated that we can use dimension reduction methods in analyzing the effects of water level fluctuations on wetland vegetation in Poyang Lake, because of the inner correlation between the above mentioned five factors. Meanwhile, previously extensive application of elevation in this field is mainly due to the comprehensive influence of different water level fluctuation factors, and only should be used in conditions with low accuracy requirement. This study provided quantitative evidence that water-level fluctuation factors drive the distribution of Carex, and may assist in making decision regarding the dam management, for the purpose of maintaining the floodplain grassland ecosystem, as well as realizing its ecological value.