Fish resources were investigated once a month from July 2017 to February 2021, and the customized gill nets, shrimp cages and fishing hooks were used as the main operating tools, analysis methods such as Index of Relative Importance (IRI), Abundance Biomass comparison Curve (ABC), similarity percentages analysis (SIMPER), non metric multi-dimensional scaling (NMDS) ecological network, temporal niche and null models were use to explore the impact of the Datengxia Dam on the fish community in Xunjiang River, Guangxi Province. During the survey, a total of 77 fish species were collected, belonging to 7 orders, 19 families and 63 genera. The Index of Relative Importance values of Ptychidio jordani, Squiliobarbus curiculus, Cyprinus carpio and Leiocassis crassilabris were between 850.59 and 2347.23, which meant that they were the dominant species in Xunjiang River. The results showed that a series of changes have taken place in the fish community of Xunjiang River after the operation of this dam:(1) The similarity percentages analysis of fish community showed that the similarity of fish community was more than 59.73% in each year, and these similarity were lower with the time going by. Besides, the number of fish species has decreased from 69 to 51; (2) The disturbance of fish community in Xunjiang River increased with time. Firstly, the results of fish Abundance Biomass comparison Curve showed that the W value decreased from 0.052 to -0.004. Moreover, the number of ecological network nodes constructed based on fish in Xunjiang River was decreasing, the structure tended to be simplified, and the stability was weakening; (3) The results of temporal niche showed that competitions between fish species has intensified, and the species pairs with the largest increase in temporal niche overlap were tilapia and Garra orientalis, Ptychidio jordani and Carassius auratus; (4) The results of the null model constructed based on the time niche overlap index revealed that the construction of Xunjiang River fish community depended on the environmental factors. After the dam impoundment, fish community has changed to a random distribution development pattern dominated by competition. To sum up, the operation of Datengxia Dam had a great impact on the fish community in the Xunjiang River, and it is urgent to take corresponding protective measures. This study contributes to environmental protection and provides a theoretical basis for the follow-up work on the recovery of fish resources of Xunjiang River and suggestions on the operation of the Datengxia Dam.