Abstract: This study investigates the effect of preparatory heat treatment on the final carburized microstructure of 18CrNiMo7-6 carburizing steel, with the aim of providing a technological reference for resolving the "performance versus cost" trade-off in mining reducer gears. Two preparatory heat treatment routes, namely normalizing plus quenching and tempering (N+QT) and isothermal normalizing (IN), were designed and cross-compared in combination with direct quenching after carburizing and reheating quenching, respectively. The characteristics of carbides, martensite, retained austenite, internal oxidation, and core microstructure under different process combinations were analyzed through metallographic observation and effective case depth measurement. The results demonstrate that isothermal normalizing produces a more uniform and finer pearlite + ferrite precursor structure compared with normalizing plus quenching and tempering, thereby providing a superior microstructural heritage for subsequent carburizing. Under identical final quenching conditions, the specimens subjected to isothermal normalizing exhibit consistently better martensite and retained austenite ratings than those of the Q&T-treated group.