The urgent demands for more excellent mechanical properties to meet higher steam temperatures for 9Cr steels have been the driving force behind the increasing research efforts on heat treatment. This paper presents a systematic study of the effect of the ultra-fine martensite (ultra-fine M) and retained austensite (RA) obtained by austempering process on mechanical properties are studied by experimental observations and phase-field simulation. The results demonstrate that austempering leads to the split and incompletion of martensitic transition, producing a mixed microstructure consisting of lathy martensite(lathy-M), ultrafine martensite(ultra-M), and retained austensite (RA). Due to the change of local chemical free energy and elastic strain energy caused by the redistribution of C element, two types (isothermal and athermal) of ultra-M form by spontaneous nucleation and interfacial migration during austempering with appropriate temperatures. Further tempering produces dispersed finer M23C6 precipitated at the interfaces of high-dislocated ultra-M than lathy-M. Meanwhile, RA decomposes into fine globular M23C6 and ferrite. Consequently, a larger amount of ultra-M and RA in quenched state results in smaller effective grain sizes and smaller carbides' average size, leading to the enhancement of precipitation strengthening, grain boundary strengthening, and impact toughness. This work establishes the potential for tailoring the strength and impact toughness of 9Cr1.5Mo1Co steel by optimizing the ultra-fine martensite and retained austensite using austempering process.