The invention discloses a steel grain refining heat treatment method for a heavy-load locomotive gear. The method comprises the steps of S1, carburization, wherein steel for the gear is placed into a carburizing furnace, after the temperature rises to the first temperature, carburization is carried out, and after the temperature drops to 860 DEG C+/-5 DEG C and is constant, the steel is cooled to the room temperature; S2, isothermal normalizing, wherein the steel is subjected to heat preservation for a hour at the temperature being 900 DEG C +/-5 DEG C, and after the steel is subjected to air cooling to be at the second temperature, heat preservation is carried out for b hour, and the steel is subjected to air cooling to be at the room temperature; S3, one-time quenching, wherein after the steel is heated to 840 DEG C+/-5 DEG C and subjected to heat preservation, the steel is discharged out of the furnace, and oil quenching is carried out; S4, high-temperature tempering, wherein after the steel is heated to the third temperature and subjected to heat preservation for c hour, the steel is subjected to air cooling to be at the room temperature; S5, secondary quenching, wherein after the steel is heated to 820 DEG C+/-5 DEG C and subjected to heat preservation, the steel is discharged out of the furnace, and oil quenching is carried out; and S6, low-temperature tempering, wherein after the steel is subjected to heat preservation for d hour at the fourth temperature, the steel is subjected to air cooling to be at the room temperature. The grain refining heat treatment method is adopted, the material structure is improved, the toughness, the impact toughness, the wear-resisting property, the fatigue strength and the anti-corrosion property of materials can be improved, and the comprehensive mechanical property of the product is good.