Abstract: The present paper investigated on changes of species composition, ecological pattern, and output of fish resources in the middle and lower Yalong River dealing with the construction of the Ertan Reservoir. Data collected by surveys on commercial fisheries and field sampling in different stages showed that the assemblages have been dominated by lentic species in stead of lotic species after the impoundment of the Ertan Reservoir. Moreover, some small invasive species have already settled down and became harmful to the native species. Habitats of native and endemic species which preferred to lotic environments have been narrowed and were restricted in upper reach of the reservoir and a few tributaries. In the first years, fishery output has obviously increased due to the impoundment of the reservoir. According to the above ecological consequences caused by the construction of the Ertan Hydropewer Plant, an integrated conservation plan including measures such as to establish river reserve, to restock artificially produced juveniles of native and endemic fish species, to make an ecological aimed operation schedule of dams, and to prevent invasion of Mien fish species should be undertaken in order to protect native fish specise from harms of the step-wise construction of dams in the Yalong River in the future.