Study aim: The primary hypotheses test whether MMIH reduces emotional distress (depression, anxiety, OCI-R) and increases life satisfaction. Secondary hypotheses focus on the effects of MMIH on reducing RNT, maladaptive meta-beliefs, psychological inflexibility, and enhancing mindfulness. The mechanism of change hypothesis examines whether changes in meta-beliefs predict improvements in symptoms and broader processes beyond mindfulness. Design: This study was a three-arm randomized controlled trial (RCT) following CONSORT 2010 and longitudinal ‎measurement at three time points: baseline, post-treatment, and 6-month follow-up ‎.‎ Settings and conduct: Participants were recruited from September 3 to May 17, 2025, using a multi-tiered referral and outreach strategy to enhance representativeness and reduce self-selection bias. Participants/Inclusion and exclusion criteria: Eligibility criteria included: (a) meeting DSM-5 diagnostic criteria for at least one disorder in anxiety disorders, depressive disorders, and obsessive-compulsive and related disorders; (b) age 18 or older; and (c) provision of written informed consent. Exclusion criteria included: (a) current or past diagnosis of psychotic disorders, bipolar disorders, severe personality disorders, substance use disorders, or any debilitating medical condition (e.g., uncontrolled chronic illness); and (b) current suicidality. Intervention groups: MMIH is a 10-week manualized group therapy (90 minutes/week), delivered by ‎certified clinicians trained in the MMIH protocol. Main outcome variables: Obsessive–Compulsive Inventory–Revised; Satisfaction with Life Scale; Meta-Inner Humor Scale; Mindful Attention Awareness Scale; Repetitive Thinking Questionnaire – 10 ' Acceptance and Action Questionnaire–II; Metacognitions Questionnaire–30; Self-Compassion Scale – Short Form; Patient Health Questionnaire – 9 item; State–Trait Anxiety Inventory