Ecotourism in islands focuses on the experience of tourists in local resources and can simultaneously protect the environment and promote community development. However, as the pattern of ecotourism becomes more diverse, administrative agencies and tourism operators may gradually ignore the original intention of ecotourism and begin to pursue a large number of targets. One of its aims is to attract more visitors to ecotourism but lose the sustainability of environmental protection. This study has applied the concept of authenticity in tourism anthropology to ecotourism activities. The purpose is to understand the authenticity of tourists when conducting ecotourism from the perspectives of objective and existential authenticity. Through the theory of planned behaviors, the influence of the perception of authenticity on the re-visited behaviors observed. The survey time take from June to July 2019 for questionnaires. The blue tears of ecotourism that rise in Mazu has mainly focused and take as an example. The paradox of sustainability and authenticity in ecotourism rethinking and discussed.