Abstract: In order to identify the gas invasion and its main controlling factors in the oil and gas reservoirs in Ziniquanzi formation of Horgos anticline, using the methods of geochemical property analysis, inclusion testing and seismic analysis, this paper studies the oil and gas reservoirs in which fractionation of gas invasion occurs, and analyzes the influences of the fractionation on the reservoirs. The study result shows that the source rocks have high organic matter abundance and persistent gas generation capability; a large number of faults and a good matching of geological conditions such as oil and gas charging time in the study area cause the large scaled gas invasion fractionation in the study area; formation pressure and vertical depth are the main factors controlling the intensity of gas invasion fractionation; the gas invasion fractionation is the direct cause of the formation of complex crude oil types such as heavy oil, low waxy condensate oil and high waxy oil