Abstract: This paper proposes a mathematical model for the simulation of natural thermal degradation of kerogen. Kinetic parameters are determined on the basis of geochemical data of Chinese sedimentary basins. The apparent activation energy E is taken as 15600 cal/mol, and the apparent frequency factor A is taken as 6.7×10⁶/m.y.. When the temperature history of source rocks is established the values of temperature index Ω would be achieved from Table 1. The evolution index S and the residual ratio a of potential carbon in kerogen could be obtained by the following equations: S_i=(Ω_i-Ω_i-1÷Ω_i,I_nα_i=-(S₁+S₂+……+S_i).where n (℃/m.y.) is the ratio of temperatures, and i is the serial number of time intervals. The a value corresponding to the upper limit of principal zone of oil formation is 0.94 and a=0.55 may be used as indicator of the peak of oil generation. As a=0.06, the vitrinite reflectance would be about 2.0%. A lot of Chinese source beds have been simulated, and the results conform to the reality of natural geological sections.