Objective:To explore the mediating effect of social support on social avoidance and distress and reproductive concerns in young breast cancer patients undergoing chemotherapy.Methods:This is a cross-sectional study. From February 2020 to December 2021, a total of 180 young breast cancer patients undergoing chemotherapy in Shandong Cancer Hospital were selected as the research objects using the convenient sampling method. General Data questionnaire, Social Support Rating Scale, Reproductive Concerns After Cancer Scale (RCAC) and Social Avoidance and Distress Scale (SADS) were used to investigate patients. Pearson correlation analysis was used to investigate the correlation among social support, social avoidance, distress and reproductive concerns in young breast cancer patients undergoing chemotherapy. Structural equation models was used to explore the mediating effect of social support between social avoidance and distress and reproductive concerns. A total of 180 questionnaires were distributed in this study, and 172 were effectively received, with an effective recovery of 95.56% (172/180) .Results:The total score of SADS of young breast cancer patients undergoing chemotherapy was (18.98±3.15), the total score of RCAC was (59.85±5.03), and total score of Social Support Rating Scale was (33.53±4.25). Pearson correlation analysis results showed that social avoidance, distress was positively correlated with reproductive concerns ( r=0.810, P<0.01), and social support was negatively correlated with reproductive concerns and social avoidance and distress ( r=-0.570, -0.612; P<0.01). Structural equation model results showed that social support played a partial mediating role between social avoidance and distress and reproductive concerns. Conclusions:Social support plays a mediating effect between social avoidance and distress and reproductive concerns in young breast cancer patients undergoing chemotherapy. Medical and nursing staff should provide more social support for young breast cancer patients undergoing chemotherapy, reduce reproductive concerns and improve quality of life.