BACKGROUND: Osteonecrosis of the femoral head (ONFH) is characterised by progressive bone death, leading to joint incongruity and eventual osteoarthritis. Various interventions have been explored to forestall disease progression and delay total hip arthroplasty (THA). Free vascularised fibula grafting (FVFG) has shown promise, particularly in the precollapse stages of ONFH. However, the potential benefits of combining synovectomy with FVFG to address synovitis in ONFH have not been systematically studied. This trial seeks to compare outcomes between patients undergoing FVFG with and without synovectomy. METHODS AND ANALYSIS: The trial is a randomised, single-centre, parallel-group trial comparing FVFG with synovectomy versus FVFG alone in patients with ONFH and synovitis. 90 participants will be randomised into two groups: synovectomy (n=45) and non-synovectomy (n=45). The primary outcome is the Harris Hip Score (HHS) change at 12 months post surgery. Secondary outcomes include HHS, Numeric Rating Scale (NRS) for pain, Depression Anxiety Stress Scales-21 (DASS-21), and EQ-5D scale assessments at intervals up to 12 months post randomisation. ETHICS AND DISSEMINATION: This trial was approved by the Human Research Ethics Committee of Shanghai Sixth People's Hospital Affiliated to Shanghai Jiao Tong University School of Medicine prior to patient recruitment (approval number: 2023-076). Results from this trial will be published in peer-reviewed journals. Results will also be presented at international conferences. TRIAL REGISTRATION NUMBER: Chinese Clinical Trial Registry (ChiCTR) Identifier: ChiCTR2300073385. Prospectively registered on 10 July 2023.