The Qiangtang basin is a large Meso - Cenozoic basin on the Qinghai-Xizang Plateau, which is a vast hydrocarbon-bearing region. Researches on oil and gas generation of Mesozoic marine source rocks have been done on the basis of a large volume of data of source rocks in the Qinghai -Xizang Plateau. According to the results of studies of the Carbonate rocksin the Qiangtang basin, zo-bitumens, which are annular or in the shape of a string of beads, are generated from foraminifers occurring in the marine source rocks. It has been shown by Micro FT-IR and simulation experiment that there is an obvious correlation between Rb of zo-bitumens and R0 of vitrinite, the former being lower than the latter when R0<0. 9%. Based on analysis of the relationship between Rb and R0, the expression R0 = 0. 6704Rb + 0. 2095 has been set up, and the correlative coefficient r2 is 0. 9653, which provides an important parameter for accurately determining the organic maturity of the Mesozoic marine source rocks on the Qinghai-Xizang Plateau.