The Pacific island countries are of great significance for China’s peripheral security and global development strategy. However，there are still very limited academic works concerning their political development，especially national identity，a key factor in affecting political stability. This paper chooses Papua New Guinea，Samoa and Tonga as three typical cases and analyzes the ethnic identity and national identity in the three countries. It finds out that the strength of national identity in these countries is related with two factors：（1）whether the ethnical identity is compatible with national identity；（2）the strength of state capacity. In Papua New Guinea，due to the lack of uniformed ethnic identity and unstable state polity，the national identity is only loosely built among citizens. In Samoa，despite the existence of a unified ethnic identity，the development of national identity is limited，subject to certain restrictions including the disturbance of powerful social groups and the relatively weak state capacity. In Tonga，due to that powerful social groups and political forces are overlapped with each other，united in the leadership of the king and the nobles，the strong ethnic identity and strong state capacity promote the development of national identity.