Mediated public diplomacy is a critical way of strategic communication for governments to influence global public opinion, especially during international conflicts. Based on the theories of mediatization and digitalization as well as the layered model of communicative space, this study investigates how the United States and Iranian embassies competed on the Chinese social media platform Sina Weibo over 13 months. Following a network ethnography approach, this study conducts longitudinal network analysis and textual analysis. The findings suggest that the macro-layer function (i.e. hashtag) facilitated horizontal dissemination of news about the US-Iran conflict from mainly China’s state-media accounts, increasing public awareness of the issue within a short time. The meso-layer (repost and follower network) and micro-layer (mention and comment) functions helped establish vertical communication facilitated by opinion leaders and crowd-enabled leaders that perpetuated public attention on relevant issues. The two embassies engaged with the Chinese public in different communicative layers. The US embassy interacted with the Chinese publics through reposts and replies, while the Iranian embassy constructed their posts in the form of replies. The Iranian embassy leveraged China’s state media agendas as narrative agency to conduct diplomatic communication by incorporating relevant hashtags in their posts. A layered model of mediated public diplomacy is proposed. The findings deepen our understanding of competing mediated public diplomacy, and contribute to the field of international communication of international conflicts.