Abstract: The dam blocks the river, not only obstructing the migration of fish, but also altering the river’s flow regime and habitat characteristics, which has a significant impact on the life and diversity of fish. In order to understand the fish diversity and resource status after the operation of Nanmu River fish passway of the Datengxia water conservancy Project, a fish survey was conducted seasonally in the Nanmu River from May 2024 to February 2025. A total of 1 983 fish individuals were collected, belonging to 5 orders, 17 families, 53 genera and 68 species. Cyprinidae was the biggest family, with 36 species accounting for 52.94% of the total species. One species listed as anational second-class protected animal and eight species endemic to the Pearl River system, and three invasive fish species were collected. The number of fish species increased and the fish composition was significantly changed, compared to the survey in 2022. The diversity indexes indicated that the overall fish diversity of the Nanmu River was at a high level. The Margalef index of the fish community in Nanmu River ranged from 4.31 to 6.51, the Shannon-Wiener index ranged from 2.11 to 3.08, and the Pielou index ranged from 0.59 to 0.85. At the sampling site S4, located at the confluence of the Dahuangjiang River and Xunjiang River, the Shannon-Wiener index, Margalef index, and Pielou index of the fish community were the highest among the 4 sampling sites, at 3.08, 6.51, and 0.85 respectively. IRI (Index of relative importance) indicated Coptodon zillii and Acheilognathus tonkinensis were overall predominant species of the Nanmu River. The predominant species varied among the four sampling sites although Coptodon zillii was the common predominant species of all sampling sites. The Cluster Analysis results showed that the fish community structures were significantly different among the different section of the Nanmu River, which were classified into three distinct groups. The ecological fish ladder inlet of Nanmu River (S1) and the Sizhu Village (S2) were classified into Group I; and the confluence of Nanmu River and Zijing River (S3) was classified into Group II; and the confluence of Dahuang River and Xunjiang River (S4) were classified into Group III. The abundance-biomass curve analysis indicated that the fish community structure at the confluence of the Dahuang River and Xunjiang River (S4) was relatively stable, and the fish community at Sizhu Village (S2) and Zijinghe River (S3) were in a moderately disturbed state, and the ecological fish ladder inlet of Nanmu River (S1) was in a severely disturbed state. The results also indicated that the fish community composition in the Nanmu River was characterized by Cyprinidae species after the operation of Nanmu River fish passway of the Datengxia Dam, While the number of fish species and diversity indices have shown an increase compared to 2022, the fish community structures across different river sections exhibited dissimilarities. Fish resources were severely threatened by invasive alien species. Although the construction of ecological fish passageways has mitigated the impact of damming, continuous optimization of the ecological flow regulation plan and monitoring of fish passage effectiveness are still necessary.