The impacts of thermal maturity on the pore size, volume, and distribution in transitional shale are recognized. Combined with previous research results, Shanxi shale of the upper Paleozoic formation in the Ordos Basin, China, was selected to identify the evolution of shale pore size and types, and clarify the factors affecting the pore development. Samples of Shanxi shale from the northeastern Ordos Basin (NOB) were selected for experimental analysis, including total organic carbon (TOC) content, vitamin reflection (R-0), X-ray diffraction (XRD) analysis, argon ion polishing-scanning electron microscopy (AIP-SEM), energy-dispersive spectroscopy (EDS), and low-pressure carbon dioxide (CO2) and nitrogen (N-2) adsorption analyses to illustrate the geochemistry of organic matter, mineral composition, pore morphology, size, and distribution. The data show that the Shanxi shale in the NOB is in the oil generation stage and has good conditions for gas generation. The interparticle (interP) and intraparticle (intraP) pores are related to mineral matrix rather than organic matter with a relatively high value of pore volume. In addition, combined with the characteristics of organic geochemistry and reservoir development of Shanxi shale in the southeastern Ordos Basin (SOB), the factors affecting shale reservoir development and the effect of thermal maturity on the evolution of the pore type and size can be explained. The presence of quartz inhibits the increase in pore volume in the low thermal maturity stage, but it is beneficial to pore preservation in the high thermal maturity stage, and different clay mineral components have different effects on pore development. Furthermore, the pore types of shale in the low maturity stage are mainly interP and intraP pores related to inorganic mineral, and organic pores are rarely developed. With the increase in thermal maturity, the number and volume of organic matter pores increase, but inorganic pores are still the main pore type. The effect of thermal maturity on the evolution of the pore type and size will enrich a basis of understanding of gas storage, transport mechanics, and fracturability that can be beneficial to the evaluation of shale reservoirs.