The remarkable development of digital and mobile devices accelerates the prevalence of both popular cultures worldwide. In this digital era, the emergence of social media enables individuals to access to a variety of information and experience innovative behaviours, such as participating in an online transcultural fandom community. As South Korea continues to export its popular culture, Hallyu, from Korean drama to Korean popular music (K-pop), the audience of the Korean Wave has expanded from Asia to other continents. Especially in those anglophone countries, the fan base has reached a noticeable scale with the help of the online fandom community. Notably, on hierarchical social media platforms, the gatekeeping effect exists in the online K-pop fandom community, and all community members perform in an established manner. In order to identify the causes of this phenomenon, this paper applies the ethnographic approach to obverse and analyse the speech of all participants within an online transcultural fandom community called /r/ bangtan on Reddit. Moderators gatekeeping power is contributed by the hierarchical relationship within the community, all community members awareness and acknowledgment of their schema, their unlimited social capital resulting from their unbiasedness to the community and interpersonal skills, superior accumulated knowledge about the object of fandom, as well as linguistic, cultural, and geographic boundaries in the transcultural context. Finally, some possible recommendations are provided for moderators to manage their online fandom communities on hierarchical social media platforms, including knowledge accumulation about celebrities and related languages, interpersonal capability development, and unbiased treatments.