During the installation process of P92 steel pipes in the ultra-supercritical units, tempering treatment is a popular method to adjust the hardness of weld joints so that the hardness can meet the installation standard. However, it is still unclear how repeated heat treatments affect the microstructure and hardness of P92 steel. Thus, P92 steel was subjected to 1~4 times of tempering, and the microstructure and hardness evolution were analysed by utilizing OM, SEM, XRD, TEM and hardness testing. The results show that P92 steel is mainly composed of lath martensite, the M 23 C 6 phase and the MX phase. The M 23 C 6 phase mainly precipitates on the interface of the lath or original austenite boundaries, while the MX phase distributes inside martensite laths. The phase composition of P92 steel has little change after different tempering times. With increasing tempering times, the width of the martensitic lath gradually increases by approximately 52.2% after 4 times of tempering. The M 23 C 6 phase at the interface obviously coarsens, while the amount gradually decreases. The main coarsening mechanism of the M 23 C 6 phase is Ostwald ripening. In addition, the contents of Cr, Mo and W alloying elements decrease in the matrix, causing the weakening of the solid solution strengthening. Consequently, the Vickers hardness of P92 steel tends to decrease with increasing tempering times.