Abstract: This paper evaluates the source rocks of the Baiyun depression in the Pearl River Mouth Basin via three aspects：abundance，type and maturity of organic matters，and sets an innovative index “Z” to comprehensively represent the abundance of organic matters.It is concluded that the source rocks in Baiyun depression are mainly good-good source rocks.The general variation law of Z value in the plane is statistically analyzed，and the source rock of the sag is superior to the raised area.Also，the ratio of thorium to uranium (Th/U) is used to analyze the ancient sedimentary environment of source rocks.The ancient sedimentary environment of source rocks in raised area is mainly oxidation，and that in sag is reducibility.From the bottom up，the reducibility of the paleo-oxygen phase of the hydrocarbon source rocks in Zhuhai is stronger than that in the Pearl River formation；the Wenchang formation is stronger than the Enping formation.By comparing the index Z and the paleo-oxygen phase，the Z value is larger in the region with the strong reducibility of the paleo-oxygen phase.The specific representation is that the higher reducibility of the organic matter，the higher the organic matter abundance of the source rock and the higher the quality of the source rock.