Background: The departure of the mature larvae of the horse stomach bot fly from the host indicates the beginning of a new infection period, and the Gasterophilus pecorum becomes the dominant species in the desert steppe, showing its special biological characteristics. The population dynamics of G. pecorum were studied to reveal the population development rule of G. pecorum in the arid desert steppe. Method: The larvae were collected and recorded in the newly excreted feces by tracking the Przewalski’s horses (Equus przewalskii), meanwhile, the larval pupation experiments were carried out under natural conditions. Results: (a) There was a positive correlation between the survival rate and the number of larvae (r = 0.630, p < 0.01), indicating that the species development had the characteristics of centralized occurrence; (b) The main periods of mature larvae discharge were from early April to early May (peak Ⅰ) and from mid-August to early September (peak II), and the larval population curve showed a sudden spike in increase and gradual decrease at both peaks; under higher temperature, the number of adults from peak II had higher survival rate, higher pupation rate, higher emergence rate and less eclosion time than that of peak I; (c) Although it has one generation a year, the occurrence peak twice annually displaying a unique “ double rhythm development ” phenomenon, which forms a “ dual rhythm parasitism ” pressure on the local host. This phenomenon is very rare in the study of insect life history, especially in the parasite epidemiology. Conclusion: The natural discharge period of the G. pecorum larvae in Kalamaili Nature Reserve (KNR) is longer than 7 months and have the potentially long term infection effect on the host. The above phenomenon is one important reason for the local equine animals to be severely infected with equine myiasis.