During the welding thermal cycle of pipeline girth welds, the weld-affected joint region—particularly the heat-affected zone (HAZ)—undergoes pronounced microstructural evolution and concomitant changes in mechanical performance. Owing to spatial variations in peak temperature, the HAZ is subdivided into multiple sub-zones with distinct gradients, and such microstructural heterogeneity is a key factor governing pipeline failure behavior. In this work, the microstructures of X80 pipeline girth welds were characterized by electron microscopy. Thermal simulation was employed to produce representative specimens for individual HAZ sub-zones, followed by Vickers hardness measurements and small punch testing (SPT) to systematically evaluate the microstructure–property relationships across different regions of the joint. The results demonstrate that thermal simulation can reproduce the microstructural characteristics of the actual welded joint with high fidelity: the phase constituents and microstructural features of the simulated coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ), and intercritical HAZ (ICHAZ) exhibit excellent agreement with those in the corresponding regions of the real weld. Mechanically, a pronounced performance gradient is observed: compared with the base metal, the CGHAZ exhibits severe softening, whereas the FGHAZ shows a recovery in hardness and an anomalous strengthening in terms of strength. Mechanistic analysis indicates that the HAZ property evolution results from the coupled effects of precipitate dissolution, grain-size variation, and the nature of transformation products. These findings validate the effectiveness of thermal simulation for assessing the performance of extremely narrow HAZsub-zones and provide a theoretical basis and experimental evidence for integrity assessment of high-grade pipelines.