Gasterophilus pecorum is the dominant horse bot fly in the desert steppe of Xinjiang, which poses a serious threat to the Przewalski's horses released to the local area. The fly lays its eggs on the grass and infects the herbivore, equines. The larvae are parasitic in the digestive tract of the host, and when they develop to the third instar, the mature larvae enter the environment with the hosts' feces to pupate and become a new epidemic focus in that year. In order to understand the distribution and characteristics of contaminated area of the G. pecorum, the fresh feces of Przewalski's horses and the sites of G. pecorum larvae were investigated in the two larvae peak periods (April-May, August) in the core habitat of released Przewalski's horses in Kalamaili Ungulate Nature Reserve, Xinjiang in 2021. The feces distribution range was calculated with Minimum Convex Polygon (MCP) and Fixed Kernel Estimator (FKE), and the larvae aggregation area and characteristics were identified with Kernel Density Estimation (KDE). The results showed that:(1) in the first and second peak periods of G. pecorum larvae population, the largest proportion (57.25% and 41.94%) of larvae was found in the grassland where the Przewalski's horses graze, followed by donkey road (the path for equines to the water source in formation), hillside and water source area. (2) The methods of Minimum Convex Polygon (95% utility probability percentage) and Fixed Kernel Estimator (95% utility probability percentage and 0.006 bandwidth) could well estimate the distribution range of Przewalski's horse feces, and the distribution range in the first peak period (60-70 km²) was slightly larger than that in the second peak period (50-60 km²). In eight water sources investigated, the three water sources, Hongliu, No. 5 and No. 6 were included in the two peak periods and a continuous and non-uniform distribution of the epidemic focus was formed with the water source as an important support point and relying on the adjacent grassland to cross spread. (3) The distribution density of G. pecorum larvae was higher in the grassland near the water source, but there was a certain difference in the spatial locations in the two peaks, with the average distance to the nearest water source being (1367±831) and (1756±1297) m, respectively. The water source of desert steppe and its adjacent grassland were the main places where the Przewalski's horses roam and gather, which was an important platform for the occurrence of G. pecorum, carrying the deductive relationship of G. pecorum-herbage-equine. The Przewalski's horses were not only the victim, but also the spreader of the parasitism, and its close relationship with the water source made it an important participant in the local myiasis epidemic. The bimodal development of G. pecorum population increased the probability of host infection and aggravated the infection degree of local equines, which is also one of the important reasons why the Przewalski's horses in this area suffer from G. pecorum constantly and severely.