Objective:To explore the mediating role of rumination thinking between demoralization and quality of life in malignant tumor patients, provide guidance and reference for helping tumor patients overcome rumination thinking and demoralization and improve quality of life.Methods:From February 2020 to June 2022, 189 patients with malignant tumors admitted to the Department of Oncology of the First Affiliated Hospital of Guangxi Medical University were selected by convenience sampling method as the research objects, and a cross-sectional survey was conducted using general information questionnaire, Demoralization Scale-Mandarin Version, Ruminative Responses Scale, Punctional Assessment of Cancer Therapy-General.Results:Among 189 malignant tumor patients, there were 102 males, 87 females, aged (43.54 ± 13.12) years old. The total score of loss of demoralization was (34.37 ± 10.34) points, the total score of rumination thinking was (41.01 ± 17.10) points, the total score of quality of life was (48.51 ± 15.41) points. The Pearson analysis results showed that the total score of demoralization in malignant tumor patients was negatively correlated with the total score of quality of life ( r = -0.502, P<0.01); the total score of rumination thinking was negatively correlated with the total score of quality of life ( r = -0.465, P<0.01), and the total score of demoralization was positively correlated with the total score of rumination thinking ( r = 0.628, P<0.01). Bootstrap mediation test results showed that ruminant thinking played a partial mediating effect between demoralization and quality of life of patients with malignant tumors, accounted for 30.9% of the total effect. Conclusions:Rumination plays a partially mediating role in the demoralization and quality of life of patients with malignant tumors, suggesting that clinical staff can improve the quality of life of patients with tumors by developing a systematic and comprehensive cognitive-behavioral intervention strategy to improve the demoralization and rumination.