Adolescent and young adult (AYA) cancer patients, aged between 15 to 39years old, suffer from long-term psychological distress, confronting low self-efficacy and various psychological problems. This study constructs a group online-based peer support intervention combined with offline activities to explore its impact on the psychological distress of AYA cancer patients. A randomized, two-arm clinical trial was conducted in which 90 AYA cancer patients were recruited. The control group (N = 45) received conventional psychological care and treatment, and the experimental group (N = 45) received 8weeks of an online peer support intervention. Outcome measures included psychological distress (Distress Thermometer, DT), anxiety and depression (Hospital Anxiety and Depression Scale, HADS), perceived peer support (Cancer Peer Support Scales, CaPSS), and readiness for return to work (Readiness to Return-To-Work Scale, RRTW). Eight-week peer support intervention was effective in improving psychological distress, anxiety, and depressive symptoms in the experimental group with statistically significant differences (P < 0.05). Time affected psychological distress, anxiety, and depressive symptoms in AYA cancer patients (P < 0.05), and there was an interaction with intervention factors (P < 0.05). The intervention has a positive effect on relieving the psychological status of AYA cancer patients. For readiness for return to work, the experimental group was in the preparation for the action-behavioral stage immediately, 1month and 3months after the end of the intervention (P < 0.01), supporting AYA cancer patients who have not returned to work to maintain optimal return-to-work readiness. The group online-based peer support intervention is popular and has good scientificity, effectiveness, and practical significance for AYA cancer patients. This study was registered at clinicaltrials.gov. (ChiCTR2100053091, registered on 10 November 2021).