Purpose <p>To determine the influence of inner strength on demoralization in patients receiving chemotherapy for lung cancer and confirm the mediating role of positive coping style.</p> Methods <p>This study was a cross-sectional study and included 210 patients from three general hospitals in China. A sociodemographic questionnaire, Inner Strength Scale, Simplified coping style subscale, and Demoralization Scale-II were applied. A structural equation model was conducted to estimate the hypothesis model of demoralization and explore the potential mediating mechanism.</p> Results <p>Inner strength was positively correlated with positive coping style (<i>P</i> &lt; 0.01). Inner strength, positive coping style, and demoralization were negatively correlated (<i>P</i> &lt; 0.01). Furthermore, positive coping style mediated the relationship between inner strength and demoralization (<i>β</i> = − 0.269 (95% CI, − 0.415, − 0.159), <i>P</i> &lt; 0.01), accounting for 37.41% of the total effect.</p> Conclusion <p>The results suggest a negative association between inner strength and demoralization in patients with lung cancer undergoing chemotherapy. Positive coping style as a mediating variable provided the impetus for the demoralization. Healthcare professionals might enhance the inner strength of lung cancer patients undergoing chemotherapy by carrying out targeted interventions and encouraging them to adopt positive coping strategies, which can reduce the level of demoralization and promote mental health.</p>