To address the issue of repeated tempering during the installation of P92 steel welded joints in ultra-supercritical units, the influence of tempering cycles on the microstructure and mechanical properties of a weld was systematically investigated. The evolution of the grain morphology and the distribution of precipitates in the P92 steel welds were analysed via OM, SEM, EBSD, and TEM characterization techniques. The results reveal that the weld microstructure primarily consists of tempered martensite, M23C6 carbides along lath boundaries, and dispersed δ-Fe phases. With increasing tempering cycles, M23C6 carbides significantly aggregate along martensite lath boundaries and preferentially nucleate and grow at δ-ferrite grain boundaries, eventually forming a continuous carbide network. These findings indicate that multiple tempering treatments regulate carbide precipitation and dislocation substructure rearrangement, leading to a gradual decrease in the microhardness and an improvement in the impact toughness. Notably, after four tempering cycles, some subgrains transform into equiaxed grains, whereas the M23C6 carbides coarsen and segregate at the grain boundaries. This phenomenon not only induces a rapid increase in high-angle grain boundaries but also significantly weakens the solid solution strengthening effect, ultimately resulting in a sharp decline in the microhardness.