Sea ice plays specific roles in the global climate system. The variation of sea-ice distribution has a great influence on the regional ocean and atmosphere in dynamical and thermaldynamical ways, thus contributing to the global climate system. In the last glacial period, there was much more sea ice than in the present day, especially in the Arctic Ocean. Owing to much colder inferior atmospheric temperature, most sea ice was perennial. But a visible seasonality created difference in the ocean–atmosphere heat flux. Sea-ice concentration always provides clues for Arctic Ocean and Nordic Sea surface circulation. The largest glacial ice-free area (along the western continental margin of Scandinavia) leads the path by which the North Atlantic Ocean surface water flowed into the Arctic Ocean 21 000 years before present. We successfully simulated the sea-ice variation and ocean circulation changes by an ocean–sea-ice coupled model, which has been well validated and proved by proxy data from marine geology research. Following the track of sea-ice drifting, we could find that most sea ice forms locally in the central Arctic Ocean in the present day; however, in the last glacial period, the east Beaufort Sea was a very import source for sea-ice creation. Except for losing heat to the atmosphere, the sensitivity analysis experiments show the redistributed land–ocean–ice-sheet in the glacial period reduced the amount of north-forwards Atlantic warmer surface water. That is also a key factor to get so much sea ice in the colder phase of the Earth’s climate. We also evaluated the influence to sea ice, from river runoffs, North Atlantic Ocean. By comparison of sea ice in glacial and interglacial Earth climate, we get a further insight into Arctic sea ice in the climate system and more information behind its variations.