Background The coping theory shows that stressful life events are associated with individuals’ psychology/behaviors, and the COVID-19 pandemic is known to have impacted individuals’ physical and mental health. Prior studies revealed that undergraduates have many sexual behavior and emotion disorders, which may be impacted during an isolation period, such as the one brought by COVID-19. However, few studies have explored the longitudinal associations between COVID-19-related stress and sexual compulsivity symptoms (SCS), and the mediating effect of emotions (i.e., depression and anxiety) on this relationship. This longitudinal study aimed to investigate these associations. Methods We employed a cross-lagged design (Time 1, 3,219 participants; Time 2, 2,998 participants) and recruited Chinese undergraduates through an online system to respond to a survey. Results Our results showed that COVID-19-related stress at Time 1 directly influenced SCS at Time 1, and there was an indirect influence via depression and anxiety at Time 1. COVID-19-related stress at Time 1 positively correlated with depression, anxiety, and SCS at Time 2, and the first could directly and positively predict SCS at Time 2. Moreover, albeit depression at Time 2 was negatively linked to SCS at Time 2, anxiety at Time 2 enhanced the effect of COVID-19-related stress on SCS. Conclusions Our findings extend the literature on SCS, showing that the higher the COVID-19-related stress, the higher the SCS, and that this effect was associated with increased anxiety in undergraduates. Thus, compared with depression, anxiety different which anxiety ought to some behavior to reduce individual inner negative emotions.