9Cr martensitic heat-resistant steel is an important candidate material for ultra-supercritical steam turbine rotors. Due to the large size of the rotor forgings， there is a greater tendency for microstructure inheritance. The microstructure and mechanical properties of 9Cr martensitic heat-resistant steel at different austenitization temperatures and isothermal annealing times were studied using optical microscopy combined with tensile and impact tests. The results showed that the austenitization temperature had a significant effect on the isothermal transformation degree of 9Cr martensitic heat-resistant steel. The austenitization temperature of this heat-resistant steel before isothermal annealing should be controlled between 950 ℃-1 030 ℃； The coarse martensitic structure formed after forging can be transformed into a ferrite+pearlite-like structure by isothermal annealing at 720 ℃， effectively breaking the microstructure inheritance and laying a good organizational foundation for subsequent quenching and tempering treatment.The effect of insulation at 720 ℃ for 30 hours-100 hours on the grain size after subsequent quenching and tempering treatment showed a weak correlation. With the extension of holding time， atomic diffusion became more complete and the distribution of microstructure became more uniform. When annealing at 720 ℃ for 50 hours to 80 hours， the material exhibited excellent comprehensive performance.This study provides theoretical basis and engineering reference for the high-performance heat treatment process design of 9Cr martensitic heat-resistant steel forgings.