In Liaozhong Depression,there were four sets of hydrocarbon source rocks.Each of them had different contribution to the oil and gas accumulation in this depression,Based on the source rock evaluation, the authors used the theory of hydrocarbon expulsion threshold and the hydrocarbon-generating potential method to research the hydrocarbon expulsion characteristics of upper Ed2,Ed3,Es1 and Es3 source rocks in the Paleogene,and evaluate the strength of their expulsion.The results show that the source rock Es3 in Liaozhong Depression contributed the most for hydrocarbon accumulation in Liaozhong Depression,is main source rock of Liaozhong Depression.