Throughout the COVID-19 pandemic, the mental health issues of university students received increasing attention. However, little is known about the effect of COVID-19-related stress on social inhibition among Chinese university students and its psychological mechanism in the context of the COVID-19 pandemic. This study aimed to explore the relationship between COVID-19-related stress and social inhibition as well as the mediating role of psychological richness. In late 2022, 1250 Chinese university students were recruited for participation in an online survey. Modified SARS-Stress Scale, Social Inhibition Questionnaire, and Psychologically Rich Life Questionnaire were used to assess participants’ COVID-19-related stress, social inhibition, and psychological richness. The Results indicate that COVID-19-related stress positively predicts social inhibition and negatively predicts psychological richness. Meanwhile, Psychological richness negatively predicts social inhibition. Mediation analysis shows that the relationship between COVID-19-related stress and social inhibition was partially mediated by psychological richness. The results indicate the significant predictive value of psychological richness when studying mental health issues.