<p>Adverse childhood experiences and rumination are both risk factors for non-suicidal self-injury (NSSI) in depression. However, the interaction among these three factors has not been well researched. This study aimed to investigate the potential mediating role of rumination between adverse childhood experiences and NSSI frequency. To this end, 276 young patients aged 15–30 years with major depressive disorder (MDD) were recruited as the research subjects. General demographic scales and clinical scales were used to assess adverse childhood experiences, rumination, levels of anxiety and depression, as well as the frequency of NSSI. Based on DSM-5 diagnostic criteria for NSSI, MDD patients were divided into two groups: the NSSI + group (<i>n</i> = 134) and the NSSI- group (<i>n</i> = 142). Mediation analysis was employed to examine the mediating role of rumination in the relationship between adverse childhood experiences and NSSI frequency. The NSSI + group exhibited higher adverse childhood experiences, rumination scores and anxiety/depression levels than the NSSI − group (all <i>p</i> &lt; 0.05). After adjusting for demographic covariates, adverse childhood experiences and different types of rumination (depression related, brooding and reflection) were positively linked to NSSI frequency over three time periods (past month/past 6 months/past year). Additionally, the depression related and brooding subtypes of rumination mediate the relationship between adverse childhood experiences and NSSI frequency. Thus, this study indicates an association between adverse childhood experiences, rumination, and NSSI. These findings suggest that heightened awareness of childhood experiences, combined with interventions targeting rumination, may help reduce the risk of NSSI.</p>