The pore structure characteristics of shale reservoirs are an intense research topic in unconventional oil and gas exploration and development. To explore the evolution of the shale pore structure at different maturity stages, the immature organic-rich shale (Ro = 0.27%, TOC = 26.27%) of the Paleogene Huadian Formation is selected for a series of thermal simulation experiments in a closed system from the immature to overmature stages. The N-2/CO2 adsorption experiment and the Frenkel-Halsey-Hill (FHH) model are used to characterize the pore development and fractal dimensions of the thermal simulation samples before and after Soxhlet extraction. The results show that most of the oil-generating stage occurs between 300 and 400 degrees C. With an increase in temperature, abundant liquid hydrocarbons will undergo secondary cracking to generate wet gas. The thermal simulation samples mainly develop wedge-shaped or slit-shaped pores. The pore size distribution mainly exists within the bimodal distributions 0.5-0.8 and 30-100 nm. The residue of liquid hydrocarbons can block some mesopores and macropores, which is shown by comparing the pore volume before and after Soxhlet extraction. Moreover, the pore structure of the samples is more complex than the pore surface during the hydrocarbon evolutionary process. The pore evolution of shale can be divided into three stages: the complex development stage of pores in the low-maturity stage, the massive development stage of pores in the middle maturity to high-maturity stage, and the uniform development stage of pores in the overmature stage. Additionally, during the entire thermal evolution of shale, residual oil and diagenesis are the main factors affecting the differential development of pores and the variation in the fractal dimension.