Abstract: This paper deals with the biodiversity study of freshwater fishes.There are 82 endemic species belonging to 15 families,60 genera in three representative branches (Nanxi River,Luzhi River and Lixiang River) of Yuanjiang River system.Each branch can be treated as a sample.Index number of β diversity are studied among three branches of Yuanjiang River.The results of β diversity analysis indicate that the difference of fish diversity between Luzhi River and Lixian River are not significant,and that is very obvious when these two rivers are compared with Nanxi River.It is less common shared number of species and genera between Luzhi River and Lixian River and this means that there is a bigger isolation in species and genera between these two rivers.It is more common shared number of species and genera between Nanxi River and Luzhi River and this means that there is a more closed relationship in species and genera between these two branches.The reasons that cause the difference of fish diversity among three branches may be probably included that degree of investigation for every branch of Yuanjiang River system,elements of environment and habitat of fish,human activities etc.However,fish diversity among three branches is not related to the length and area of river.Finally,a suggestion for fish conservation is presented.