BACKGROUND: Tai Chi (TC), as one of mild to moderate exercise therapies specifically recommended by clinical practice guideline from the American College of Physician, is a viable option for chronic non-specific low back pain (CNLBP) treatment. Nevertheless, limited studies focused on the effect of different weekly frequencies of TC in elders with CNLBP. This superiority study aims to compare the differences of TC with different weekly frequencies in elders with CNLBP on the premise of proving its effectiveness, and identifying whether mindfulness mediates the effect of TC on treatment outcomes. METHODS: In total, 284 senior citizens with CNLBP will be recruited in this single-centre, randomised, single-blinded (outcome assessors, data managers and the statistician), parallel controlled trial. Participants will be randomly divided into either one of three TC groups (1, 3, or 5 sessions/week, on the basis of weekly health educational lectures) or weekly health educational lectures, sustaining for 12 weeks, followed by 12 weeks of follow-up after the end of intervention. The primary outcome (the changes of LBP intensity at rest) will be measured at baseline before randomisation and immediately after the completion of weeks 4, 8 and 12 of the intervention, and the end of follow-up (week 24) using the visual analogue scale (VAS, 0-10 cm) to put a mark on the VAS scale to show how severities of their average low back pain have been over the past 24 h. Secondary outcomes, including Beck Depression Inventory-II, Pain Catastrophising Scale and Five Facet Mindfulness Questionnaire, Oswestry Disability Index and Short Form-36, will be measured at baseline and immediately after the completion of week 12 of the intervention and end of follow-up. The intention-to-treat and per-protocol principles will be used to analyse outcomes with a setting at α = 0.05 as statistical significance. DISCUSSION: This comprehensive and detailed protocol will be the first trial to compare the effectiveness of different weekly frequencies of TC in elders with CNLBP. The outcomes may provide valuable data about the choice of the ideal number of sessions to further normalise the application of exercise for clinicians. TRIAL REGISTRATION: Chinese clinical trial registry ChiCTR2200058190 . Registered on 1 April 2022.