Abstract: Lacustrine source rocks are widely developed in the second member of Bayingebi Formation of the Lower Cretaceous (K₁b²) in the Yin'gen-E'ji'naqi Basin, and they are the main petroleum source rocks in the basin. Pyrolysis experiments show that the total hydrocarbon yield of the samples is 265-798 mg/g, the oil yield is 264-779 mg/g in the "oil window" (EasyR_o of 0.7%-1.5%), and the yield of gaseous hydrocarbon during the main gas generation stage (EasyR_o>1.5%) is 191-583 mL/g, which indicates that the source rocks not only have a high oil generation potential, but also have a high gas potential. The variation of hydrocarbon products during the pyrolysis experiments allow the thermal evolution of organic matter of source rocks to be divided into four main stages. The first stage is kerogen pyrolysis and oil generation stage, with an EasyR_o of 0.57%-0.8% (the initial EasyR_o of pyrolysis experiment is 0.57%). Kerogen pyrolyzes into oil. The second stage is the condensate oil generation stage, with an EasyR_o of 0.8%-1.5%. Early oil cracks into light hydrocarbons and kerogen cracks into gas. In the third stage, oil cracks into gas, with an EasyR_o of 1.5%-3.3%. The fourth stage is the gas cracking stage, with an EasyR_o more than 3.3%. C_2-5 cracks into methane.