The research investigated the prevalence of nonsuicidal self-injury (NSSI) during the COVID-19 outbreak and identified the psychosocial risk factors among junior high school students in Taiwan. Cross-sectional design was applied and 1,060 participants (Mage = 14.66, SD = 0.86 years) were recruited into the study. The prevalence of NSSI was found to be 40.9% (95% confidence interval, 37.9%-43.9%) during the COVID-19 outbreak. The results suggested that the self-injurers group were mostly female, and scored significantly higher in neuroticism, depression, impulsivity, alexithymia, virtual social support, dissatisfaction with academic performance, and lower in subjective wellbeing, self-esteem, actual social support, and family function than the non-injurers group. In addition, high neuroticism, low self-esteem, high virtual social support, high impulsivity, and high alexithymia were independently predictive in the logistic regression analysis. The principal results of this study suggested that NSSI was extremely prevalent among adolescents during the COVID-19 outbreak, and in particularly, personality and virtual environment risk factors and enhancing self-esteem should be the focus of NSSI preventive strategies when targeting this age population. Our results provide a reference towards designing NSSI prevention programs geared toward the high school population during the COVID-19 pandemic.