This research grounded on Qian-duei, Zhong-duei, Hou-duei and Xiangfong-duei areas, aims to collect household register materials during the Japanese-Occupying Period from census administration office of Linluo(麟洛), Changjhih(長治), Tsutian(竹田), Neipu(內埔) and Wanluan(萬巒) Township in Pingtung County. This researcher uses quantitative and qualitative methods; counts rate of exogamy and that of adoption from other ethnic groups, interviews related persons. Research findings are as follows. Firstly, there are some reasons to explain the changing boundary of the ethnic group through quantitative and qualitative aspects. One is adoption from other ethnic groups. The other is exogamy. The changing effect of ethnic group boundary caused by adoption from other ethnic groups is better than that by exogamy. Secondly, the hypothesis of variation of Hakka from quantitative and qualitative aspects is produced. There are always some reasons to cause Hakka population variations. Decreasing Hakka population by adoption from other ethnic groups is more obvious than decreasing Min-nan and Ping-pu population by adoption from Hakka groups. Hakka maybe change into ”invisible presons”, Pin-pu, Paiuan, and Min-nan persons by adoption from other ethnic groups. Pin-pu maybe change into Min-nan persons by exogamy. Min-nan has more advantages by adoption over other ethnic groups and by exogamy at the same times. Thirdly, we describe the plural ethnic identity of adopted Hakka by other ethnic groups and its performance. Some people considers themselves Hakkanese, change their ethnic identity under different life chances in one generation or different generations. Another ethnic identity is non-Hakka, different from his father's. All that is plural ethnic identity, not only related to blood relationship, but also to person's social, political, economic and cultural context.