The invention discloses a spur gear profile modification method based on thermal elastic deformation of gear teeth and deflection deformation of a gear. The spur gear profile modification method comprises the following steps: 1, establishing a three-dimensional model of a gear based on UG; 2, introducing the gear three-dimensional model into finite element software, and analyzing thermal elastic coupling deformation of the gear under a working condition; 3, establishing an overall three-dimensional model of a gearbox based on UG; 4, introducing the three-dimensional model of the gearbox into the finite element software, and analyzing stress deformation of the gearbox under a working condition; 5, analyzing deformation of a tooth surface in different directions because of comprehensive deformation of the gearbox, and analyzing deformation characteristics; 6, taking influence characteristics of the thermal elastic deformation and deflection deformation inside different planes of the gear to tooth surface stress into profile modification; and 7, taking the thermal elastic deformation and the deflection deformation of the tooth surface inside a tangent plane into account, acquiring a profile modification curved surface equation, and performing profile modification. By adopting the method, the modification precision of a spur gear working under a high-speed heavy load working condition is improved, the uniform distribution situation of a tooth surface load is effectively improved, and the service life of the gear is prolonged.