This work is a study of the interannual variability of Alaskan Coastal Water (ACW) supplied into the Arctic Ocean during the summer season. Based on hydrological data obtained during ten Chinese National Arctic Research Expeditions conducted in the summer from 1999 to 2019, the expansion of area, volume, thickness, and heat content of ACW in the southern Chukchi Sea and northern Bering Sea have been calculated for the first time, demonstrating the presence of substantial interannual variability. From 1999 to 2019, in general, each of the fundamental parameters showed two stages, namely 1999-2008 and 2010-2019, with the latter being at a higher value than the former. We repeatedly surveyed meridional hydrographic/velocity sections in the vicinity of the Bering Strait to verify that the changes in the water mass flowing into the strait are the main factors affecting water parameters in both the northern Bering Strait and the southern Chukchi Sea.