The overpressure caused by hydrocarbon generation is the main driving force of hydrocarbon migration, the research on its mechanism is gradually becoming quantitative. The source rocks of the Cambrian Qiongzhusi Formation in the Gaoshiti-Moxi area of the Sichuan Basin have experienced the oil generation phase from the end of the late-Permian to the end of the mid-Jurassic, the oil cracking phase from the end of the Jurassic to the early-Cretaceous and the kerogen gas phase later. Considering the overall compressibility of the source rock and the oil component, a quantitative model for the overpressure caused by the continuous process of hydrocarbon generation from sapropel kerogen was established. The pressure evolution calculated by the model for the oil production showed that, from the end of the Permian to the end of the Triassic, the initial stage of oil generation, the excess pressure increased slowly. From the early-Jurassic to the beginning of the mid-Jurassic, the oil window stage, the excess pressure accumulated rapidly with a pressure coefficient up to 2.4. From the beginning of the mid-Jurassic to the end of mid-Jurassic when the oil generation ended, the overpressure reduced slightly. Calculated by the model for oil cracking, from the end of the mid-Jurassic to the beginning of the early-Cretaceous, the overpressure increased first and then decreased subsequently. In the process of oil cracking, the episodic hydrocarbon-expulsion developed in the Qiongzhusi Formation until the pressure balance was reached with a pressure coefficient of about 2.2. In addition to the types and abundance of organic matter and the thermal history, the sealing ability of the source rock and the components of cured oil are also the main factors that affect the calculation accuracy of the quantitative model. The better the closure of source rock, the greater the overpressure accumulated during the oil generation, and the higher the oil saturation retained in the source rock. The lower the sulfur content contained in the oil, the higher the conversion rate of oil to gas was and the greater the pressure formed.