In order to understand blue sheep (Pseudois nayaur) population in the Helan Mountains, Ningxia Hui Autonomous Region, we carried a blue sheep population survey using line transect method from August and December, 2020 to January 2021. We estimated the sheep population size and density, and analyzed its population structure by the Distance 1.0.2 in R 4.0.1. The results showed that the population size of blue sheep in summer was 5176 (2554-10490) and the population density was 2.674 (1.320-5.420) individual/km²; the population size in winter was 15752 (7294-34017) and the population density was 8.139 (3.769-17.58) individual/km². Blue sheep were mostly observed within 0-200 m of the vertical distance from the transect line. In summer, the main group form of the sheep was female and juvenile group (52.17%); In winter, it was mixed group (26.60%). The average group size of blue sheep was (6.261±8.023) in summer and (4.064±4.229) in winter. The difference of group size was significant between two seasons (P=0.010), ranging between 1 to 5, with a maximum of 47. In summer, male, female and juvenile accounted for 11.27%, 56.74% and 31.99%, respectively; In winter survey, male and female accounted for 29.61%, 50.15%, and juvenile accounted for 20.24%. There was significant difference in composition between summer and winter (P<0.001).