In order to investigate the current population status of blue sheep (Pseudois nayaur alashanicus) in Helan Mountain National Nature Reserve of Inner Mongolia, field work was conducted in the winter of 2017 and 2018 and spring of 2018 and 2019 by using the line transect method. Distance sampling (v1.0.2; R 4.0.3) was used to analyze the observed data to estimate the population size, density and structure of blue sheep in study area. Our results showed that a total of 6299 were observed, of which the highest population size was about 2654 (2230-3161) (The 95% quantity confidence intervals are in parentheses) in the winter of 2018, and the population density was 3.921/km²(3.293-4.668/km²). Compared with the monitoring data of the same season in 2003, it is found that its population has increased rapidly in the past 15 years, and the population density has increased by approximately 53.17%, with an annual average increase of 3.54% in study area. Meanwhile, different group types of blue sheep were monitored and showed significant differences in variant seasons (P<0.001). The mixed group was the main group form of winter (88.03%) and spring (80.95%), and its number in winter was more than that in spring, and the lowest frequency of occurrence was single individual, which lives alone both in winter (1.99%) and in spring (2.86%). There was a significant difference in the population size between winter (13.439±12.085) and spring (9.011±8.610) (P<0.001), and the population size of blue sheep was mostly 1-10 in survey season. The ratio of adult/juvenile and female/male varied from 1.411 to 2.673 and 0.934 to 1.469 in different seasons respectively, which indicated a significant seasonal difference in population composition and structure (P<0.001). Based on the results obtained in current study, we demonstrate that the population structure of blue sheep varies significantly among different seasons. In common sense, climate and resources are important drivers of species development, therefore the further study should focus on the factor of phenology and resources.