Objective: To better understand the effectiveness of a single-session positive cognitive intervention and uncover the possible mechanisms. Participants: Participants were 79 first-year students (aged 17-21) who volunteered for the intervention during the 2016 fall semester. Methods: Participants were randomly assigned into the intervention group and the control group. All participants completed pretest, post-test, 1 week, and 3-month follow-up tests. Thriving, negative emotional states, strengths knowledge were assessed. Results: Results showed significant increases of thriving in the intervention group at post-test and after 1 week, and significant decreases of negative emotions at the post-test and after 1 week and 3 months. Strengths knowledge fully mediated the intervention effect on thriving at post-test. Conclusions: The single-session positive cognitive intervention could be a responsive and effective approach to promote first-year students mental health. More attention should be paid to mechanisms of the single-session positive cognitive intervention so as to optimize the effects of the single-session positive cognitive intervention.