In the context of the Yangtze River protection, the lakes in the middle and lower reaches of the Yangtze River floodplain have experienced a transition from intensive pen-culture fisheries to fishing ban and diversity conservation. These lakes not only support high fish diversity, but also serve as important fishery production bases in China. Integrating and coordinating biodiversity conservation and sustainable fishery development requires clarifying the relationship between fish diversity and community stability. In this study, a systematic survey was conducted to collect the fish community in Lake Shengjin during high water seasons and low water seasons before (2017) and after (2019) pen-culture ban (PCB), respectively. We used three diversity index (Shannon-Wiener diversity index, Simpson dominance index and Pielou evenness index), three functional diversity index (functional richness index (FRic), functional dispersion index (FDis), and functional evenness index (FEve)), and six functional traits (maximal length, growth rate, life span, first sexual maturity time, first sexual maturity length, trophic level) to explore the differences in stability level and drivers of stability of fish communities before and after PCB in terms of both species and function. The results showed that: (1) dominant species changed from Hypophthalmichthys molitrix and Hypophthalmichthys nobilis before PCB to Xenocypris davidi and Culter alburnus after PCB. (2) One-way PERMANOVA results showed that community stability (temporal stability index (TSI), asynchrony (ASY)) was significantly higher before PCB than after PCB. (3) Results of multiple linear stepwise regression showed a positive correlation between community stability and time to first sexual maturity time and a negative correlation with growth rate before PCB. It may be related to the functional traits of the dominant periodic strategy fish before PCB. Community stability was positively correlated with growth rate and negatively correlated with life span and first sexual maturity time after PCB. It may be related to the functional traits of the dominant opportunistic strategy fish after PCB. In addition, community stability was positively correlated with the evenness index (Pielou) before and after PCB, and communities with higher evenness may support community stability through the insurance effect between species. By analyzing and comparing the differences and relationships of fish community stability and diversity before and after PCB, this study can provide theoretical basis and data support for the conservation and utilization of fish resources in Lake Shengjin and other floodplain lakes.