Introduction <p>Low socioeconomic status (SES) has been shown to be associated with more internalizing problems, but the mechanism of this relationship has not been investigated in Chinese late adolescents.</p> Methods <p>High school students (<i>N</i> = 780, 54.45% girls) were recruited to complete questionnaires to assess SES, anxiety, depression, and cognitive fusion. We tested the hypotheses that this association would be mediated by the psychological insecurity and moderated by their cognitive fusion.</p> Results <p>Regression-based analyses indicated that (1) psychological insecurity fully mediated the relationship between SES and internalizing problems; (2) late adolescents with high vs. low cognitive fusion showed a stronger positive association between psychological insecurity and internalizing problems.</p> Discussion <p>The results highlight the important effects of psychological security and family resources on late adolescents’ mental health and give implications for interventions aimed to reduce adolescent internalizing problems through acceptance and commitment therapy.</p>