Abstract: Under the theoretical framework of Critical Discourse Analysis (CDA), this paper aims to reveal the construction of China's national image in the U.S. media and the diachronic changes during COVID-19 pandemic by using corpus-based Usage Fluctuation Analysis (UFA) method.It carries out a diachronic analysis of China-related news in U.S. web media in 2020, expecting to uncover the relationship between the longitudinal discourse change and the political, social and ideological background.The results are as follows: First, in the context of COVID-19, the U.S. media cover a variety of China-related topics, which are characterized by long-term stability, short-term focus and periodic repetitions, but the anti-China tone remains unchanged.Second, the dominant diachronic trend is mainly influenced by its domestic political activities and ideological changes.Usage Fluctuation Analysis method is effective for the description of discourse changes on a specific topic within a specific period of time.However, the qualitative analysis of underlying discourse semantics, pragmatic structure and socio-cultural background is still required to obtain a more explicit interpretation.