Abstract: Compared with first-year ice, multi-year ice has greater thickness and higher albedo, which significantly influences the reduction of Arctic sea ice extent during the summer and has far-reaching implications for global climate change. This study aims to investigate the changes in the extent of Arctic multi-year ice over the past two decades and its impact on climate. Therefore, a multi-year ice concentration retrieval algorithm, based on matching QuikSCAT and ASCAT data, was developed to analyze the time series of multi-year ice area changes from the winter of 1999/2000 to 2017/2018. The study reveals that the extent of Arctic multi-year ice has sharply decreased since the 21th century, with an average annual reduction of 0.16×10⁶ km². Furthermore, the age of Arctic multi-year ice has been steadily decreasing, with a rate of −0.22/a (significant at 95% confidence level) over the past two decades. This trend reflects the thinning of Arctic multi-year ice, making it more susceptible to melting during the summer. Further analysis shows that the rate of multi-year ice area reduction is nearly the same in the Pacific and Atlantic sectors, at 91×10³ km²/a and 86×10³ km²/a (significant at 95% confidence level), respectively. The findings of this study provide a foundation for further investigating the rapid retreat mechanisms of Arctic sea ice in the context of global warming.