For the sun gear shaft in wind turbine gearbox, the deformation will occur on the inner spline because of the thermal inserting of ring gear. So, the spline pair should be tooth relieved to avoid decrease of backlash and conflict of copulate parts. Based on the finite element analysis results, the deformation caused by the thermal inserting on the inner spline was calculated, and the theoretical tooth relieving curve was got. The practical thermal inserting deformation on the spline was measured. And the results indicate that the measuring deformations are in good consistency with the deformation calculated by finite element analysis method. The spline deformation and tooth relieving calculation method can provide a reference for the tooth relieving of the spline on long structure sun gear shaft in 1.5MW wind turbine gearbox.