Abstract: The purposes of this paper are, firstly, to explore the role of boron in studying ancient sedimentary environment, secondly, to use salinity as a parameter in the evaluation of source rocks. The works on Cretaceous-Tertiary Sequences in Qianjiang depression of Jianghan Basin indicate that in the study of ancient environment, boron in clay minerals is an effective indicator for salinity. For both the entire sedimentary sequence and the individual rhythms, boron can indicate the relative variations of salinity. Since the source of lacustrine sediments may de multiple, the content of boron in clay minerals of terrestrial sediments may also be rather high. Therefore, for distinguishing whether it is of marine or nonmarine facies, several indicators should be utilized together. The relationship between salinity of the sections and their source rock indicators shows that the sediments favorable for oil generation in a salt-bearing basin are the beds with low salinity and the rhythmic beds strongly desalinated. However, as to the individual rhythms, the argillaceous rocks below the saliferous sections representing the most desalinated environment are most favorable for oil generation, especially the oil shale at the upper and middle parts.