<p>The fluctuations in the krill population profoundly impact marine ecosystems and are key indicators of ecosystems in the Southern Ocean. In this paper, we analyzed the basic biological characteristics of Antarctic krill in the Amundsen Sea and studied its age structure using the Bhattacharya’s method based on the 38th–39th Chinese National Antarctic Research Expeditions. The results revealed that the Antarctic krill in the Amundsen Sea may have been a spawning population during the survey period. The mean length (total length, AT) and weight were 42.38&#xa0;mm and 0.72&#xa0;g, respectively, and the length‒weight relationship was <i>W</i> = 9.62966E-6<i>L</i>^2.95612 (<i>R</i>^<i>2</i> = 0.92769, N = 921). The diameter of the compound eye varied from 0.29&#xa0;mm to 1.23&#xa0;mm, and the mean was 0.78&#xa0;mm. A Pearson correlation test revealed a strong correlation between length and compound eye diameter (<i>P</i> &lt; 0.001, N = 802). The proportions of juveniles and adults were 15.96% and 84.04%, respectively; the female-to-male ratio among adults was 2.69:1; the gravid female group was 23.76%, and the non-gravid female group was 76.24%. There was a significant difference in the relationship between length and weight among the juvenile, adult female and male groups and between the gravid female and non-gravid female groups. The results of the age structure composition of Antarctic krill based on length mixture distribution analysis is superior to those based on compound eye diameter mixture distribution analysis. The age group of the Antarctic krill ranged from 0 to 4⁺ years old, and the majority were 3⁺ to 4⁺ years old (63.96%). The age groups of the adult females ranged from 1⁺ to 4⁺ years old, with 3⁺ to 4⁺ years old predominating (81.99%), and the adult males were 1⁺ and 3⁺ to 4⁺ years old, which included mainly groups that were 3⁺ to 4⁺ years old (86.60%).</p>