Semi-open hydrous pyrolysis experiments on coal-measure source rocks in the Xihu Sag were conducted to investigate the carbon isotope evolution of kerogen, bitumen, generated expelled oil, and gases with increasing thermal maturity. Seven experiments corresponding were conducted at 335 °C-575 °C, while other experimental factors, such as the heating time and rate, lithostatic and hydrodynamic pressures, and columnar original samples were kept the same. The results showed that the simulated temperatures were positive for the measured vitrinite reflectance (Ro), with a correlation coefficient (R2) of 0.9861. With increasing Ro, lower maturity, maturity, higher maturity, and post-maturity stages occurred at simulated temperatures (Ts) of 335–360 °C, 360–400 °C, 400–480 °C, and 480–575 °C, respectively. Moreover, the carbon isotopes of kerogen, bitumen, expelled oil, and gases were associated with an increased Ro. Ignoring the intermediate reaction process, the thermal evolution process can be summarized as kerogen0 (original) + bitumen0 (original) → kerogenr (residual kerogen) + expelled oil (generated) + bitumenn+r (generated + residual) + C2+ (generated + residual) + CH4 (generated). Among these, bitumen, expelled oil, and C2-5 acted as reactants and products, whereas kerogen and methane were the reactants and products, respectively. Thus, the reaction and production mechanisms of carbon isotopes can be obtained based on their changing degree and yields in kerogen, bitumen, expelled oil, and gases. Furthermore, combining the analysis of geochemical characteristics of the Pinghu Formation coal-oil-type gas in actual strata with these pyrolysis experiments, it was identified that this area also had substantial development potential.