The species Prosopocoilus reni Huang and Chen was discovered endemic to Hainan Island, the smallest and southernmost province of the People's Republic of China. The species is very similar to the widely distributed Prosopocoilus blanchardi (Parry) in China. In this study, we used the integrated approach to analyze the phylogenetic, genetic, geographic, and morphological data among Prosopocoilus reni, P. blanchardi, and some allied taxa from 43 specimens. The phylogenetic reconstruction indicated that P. blanchardi had divided into the eastern and western clades in China and P. reni was embedded in the two clades. The genetic distance implied that P. reni should not be a full species but an island population of P. blanchardi with deeply genetic isolation. The divergent time estimation further supported that P. reni was divergent from the eastern clade from Continental China, similar to another island population from Taiwan Island. The morphological comparisons among P. reni, P. blanchardi, and other allied species showed that the key diagnosed character of P. reni should be phenotypic variation as a tropical island population. Therefore, the species P. reni was proposed as a new junior synonym of P. blanchardi based on the integrated taxonomy analysis.