<p>Seasonal migration among retired individuals is increasingly common in China. Many seasonal retired migrants move to Hainan Island during the winter months and return to their original places in the warmer months. This study examines the changes in the health-related quality of life (HRQoL) among seasonal retired migrants during their migration and identifies potential effect modifiers of these changes. A prospective cohort study with one-year follow-up was conducted in Wuzhishan City, Hainan, involving 884 seasonal retired migrants who completed baseline and two follow-up investigations. HRQoL was measured using the EQ-5D-5L instrument. Fractional regression models were employed to analyze HRQoL changes and explore effect modifications by demographic, behavioral, and other health-related characteristics. The median age was 68 years, with 58.3% being female. HRQoL significantly improved when migrants moved to Wuzhishan City, with EQ-5D utility increasing by 0.058 (<i>p</i> &lt; 0.001) and EQ-VAS score rising by 4.47 (<i>p</i> &lt; 0.001) compared to their original places. Chronic conditions, baseline sleep quality, and changes in physical activity and sleep quality during migration were significant modifiers of HRQoL changes. Participants with chronic diseases experienced greater improvements in HRQoL. Improved sleep quality and increased physical activity during migration were also associated with greater improvements in HRQoL. This study reveals the associations between Chinese seasonal migrants’ HRQoL and places, whereas the migrants possess better HRQoL in Hainan Island compared to that in their original places during seasonal migration, modified by several characteristics. These findings contribute to the broader understanding of the geography of well-being and offer practical implications for policymakers and public health professionals.</p>