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Introduction As populations age and climate variability intensifies, seasonal migration has emerged as a strategy among older adults to reduce exposure to climatic extremes and optimise living conditions. In China, millions of older adults migrate annually between cold northern and tropical southern regions, providing a natural setting to examine the health effects of seasonal migration. The Frigid-Tropical Migratory Population Health (ftMPH) cohort was established to assess the short- and long-term health effects of this migration and the underlying biological, behavioural and social pathways.Methods and analysis The ftMPH cohort is a prospective, dynamic cohort jointly conducted in Heilongjiang (a cold region) and Hainan (a tropical region) in China, with a planned sample size of approximately 26 000 participants. Adults aged ≥60 years are recruited and classified into four subcohorts: cold-origin seasonal migrants, cold-region residents, tropical-origin seasonal migrants and tropical-region residents. Baseline assessments include questionnaires, clinical examinations, biospecimen collection and environmental exposure measurements. Each migration cycle is assessed at predefined time points spanning the departure and return phases. Migration exposure will be updated longitudinally at each follow-up interval.The primary outcome is the incidence of major cardiovascular disease events, including non-fatal myocardial infarction, coronary revascularisation, stroke and cardiovascular mortality. Secondary outcomes include non-cardiovascular mortality, chronic disease events, healthcare utilisation and ageing-related measures. Time-to-event analyses will be performed using Cox proportional hazards models. Inverse probability of treatment weighting (IPTW) will be used to control for baseline confounding, with stabilised weights and covariate balance assessed using standardised mean differences. Longitudinal changes in repeated measures will be analysed using mixed-effects models.Ethics and dissemination Ethical approval was obtained from the Ethics Review Committee of Harbin Medical University (approval No HMUIRB2024029PRE) and the Ethics Review Committee of Hainan Medical University (approval No HYMLL-2025-102). Findings will be disseminated through peer-reviewed publications, conference presentations and policy or technical reports.Trial registration number The ftMPH cohort is registered with the Chinese Clinical Trial Registry (ChiCTR2500102337; registered on 13 May 2025).