Abstract: Three oil and gas reservoirs have been discovered in the middle and deep layers in the southwest zone of Bozhong sag, and their phase distribution of hydrocarbon is complicated.In this paper, the phase distribution characteristic of middle and deep hydrocarbons in the southwest zone of Bozhong sag and its controlling factors are analyzed comprehensively based on the geological and geochemical conditions for the formation and distribution of oil and gas reservoirs.It is indicated that the phase behavior of hydrocarbon in Caofeidian 18-2 structure and Bozhong 13-1 structure in the northern part of southwest zone of Bozhong sag is a condensate gas reservoir with oil rim and a high volatile oil reservoir, respectively, and that in Bozhong 19-6 structure in the southern part is a condensate gas reservoir.The phase behavior of hydrocarbon is mainly controlled by differential thermal evolution of source rocks, gas invasion and temperature-pressure condition.The hydrocarbons in Caofeidian 18-2 structure and Bozhong 13-1 structure are mainly generated by the adjacent Ed3 source rocks, and the hydrocarbon in Bozhong 19-6 structure are mainly derived from the adjacent source rocks of Shahejie Formation.The difference of thermal evolution degree between Ed3 source rocks and Shahejie Formation source rocks playsa decisive role in the difference of hydrocarbons, and it is the main reason for the current phase behavior of hydrocarbon.Gas invasion is the key factor influencing the phase behavior of hydrocarbon, and the gas invasion in Bozhong 19-6 structure is much stronger than that in Caofeidian 18-2 structure and Bozhong 13-1 structure in the late stage.And temperature and pressure are the important factors influencing the phase behavior of hydrocarbon.