The source and formation mechanism of the colorless condensate oil produced in Well CBX394 on the east slope of Chengdao area are not clear. According to the feature analysis of light hydrocarbon composition，biomarkers，carbon isotopes，and adamantanes，the source of condensate oil and gas in Well CBX394 was identified and the formation mechanism was discussed. The results revealed that such colorless condensate oil is similar and relevant to the light-yellow condensate oil. The former is the product of the hydrocarbon source rocks at the high-maturity stage of Es3 in Bozhong Sag，and the light-yellow condensate oil is produced because the colorless condensate oil is mixed with a small amount of crude oil from the hydrocarbon source rock of Ed at the mature stage during production. Natural gas is the associated gas of the same source as condensate oil，and its maturity is slightly higher than that of condensate oil. On the whole，natural gas charging is featured by“early oil and late gas”. The light hydrocarbon index confirms that the evaporative fractionation of condensate oil is not obvious，and the mature thermal evolution and migration fractionation are the main mechanisms for the formation of condensate reservoirs.