The invention discloses a heat treatment technology for reducing deformation of a spiral bevel gear for a hay mower and belongs to the field of heat treatment technology. The technology comprises the following steps: 1) preparation of the spiral bevel gear by the adoption of a SCM415 blank; 2) crating; 3) front cleaning; 4) carburizing and quenching: temperature equilibrium at 850 +/- 5 DEG C for 20 +/- 1 min, and carbon potential is 0.65 +/- 0.025%; intensive carburizing and thermal insulation at 880 +/- 5 DEG C for 15 +/- 1 min, and carbon potential is 0.8 +/- 0.025%; diffusion and thermal insulation at 880 +/- 50 DEG C for 15 +/- 1 min, and carbon potential is 0.8 +/- 0.025%; quenching and temperature equilibrium at 840 +/- 5 DEG C for 20 +/- 1 min, and carbon potential is 0.65 +/- 0.025%; and cooling in 130 +/- 10 DEG C of quenching oil; 5) post cleaning; and 6) tempering at the temperature of 175 +/- 5 DEG C for 150 +/- 10 min. According to the invention, deformation of the spiral bevel gear after heat treatment is reduced, and first pass yield for heat treatment can reach 100%.