Background: The outbreak of COVID-19 has been a big challenge for senior high school students in China who are facing tremendous pressure of the highly competitive College Entrance Examination. Methods: To evaluate the psychological impact of the event in the population, we conducted an anonymous online survey among senior high school students in China between 26 Feb and 4 March, 2020. Information collected included demographic characteristics, attitude toward medical study, infection of COVID-19 in acquaintances, anxiety symptoms evaluated using the GAD-7, and health literacy level measured using the IDSHL. Results: Of 21,085 participants, 3,575 (17.0%), 943 (4.5%) and 448 (2.1%) reported with mild, moderate, and severe anxiety. Female, higher academic year, worse self-evaluated academic performance, negative attitude toward medical study, living in Hubei province and having acquaintance infected with COVID-19 were significantly associated with anxiety level, while higher education level of mother and higher IDSHL score were associated with a lower risk. The score of IDSHL, particularly of the domain "infectious disease prevention", was associated with the GAD-7 score in a linear pattern (beta=-0.0371, p<0.01). Limitations: Limitations included the cross-sectional study design unable to infer the casual relationship, anonymous survey, selection bias and self-reported anxiety disorder levels. Conclusions: The results suggested that COVID-19 outbreak may increase anxiety level in senior high school students in China. The anxiety related factors observed in this study may help to identify vulnerable individuals and develop interventions.