The invention provides a machining method of reducing deformation of a helical gear after carburizing and quenching. The machining method comprises the following steps of machining a helix angle variable beta' of reserved reverse deformation on the helical gear before carburizing and quenching of the gear is carried out, and namely before carburizing and quenching process, a machining helix angleof the helical gear is equal to a designed helix angle beta minus a helix angle balance beta' of reverse deformation; and the helix angle variable beta' is set according to the designed helix angle beta of the gear, an axial effective analysis range H of the gear, and axial heat treatment deformation amount M in the processes of gear hobbling or gear shaping. According to the machining method of reducing deformation of the helical gear after carburizing and quenching, on the basis of the law of reverse deformation, the helix angle variable is used for offsetting the distortion amount generatedby heat treatment. The helix angle of the gear after carburizing and quenching meets process requirements, thus ensuring that the deformation of upper and lower sections is not out of tolerance in subsequent axial finish machining after carburizing and quenching of the gear, and therefore the workload of subsequent finish machining as well as the cutting amount of a carburized layer can be reduced.