Provided is an assembly structure of sun gear shaft and spline shaft of a gearbox for wind turbine. The assembly structure relates to the technical field of gearboxes for wind turbine and includes a sun gear shaft, a spline shaft, a mechanical pump drive gear, and a distance ring. The spline shaft is a hollow shaft. One end of the sun gear shaft is inserted into the spline shaft. The one end of the sun gear shaft inserted into the spline shaft is connected to the spline shaft through a spline pair. The mechanical pump drive gear is mounted at one end of the spline shaft. The inner diameter of the mechanical pump drive gear is less than the inner diameter of the spline shaft. The distance ring is disposed in the spline shaft. One end of the distance ring abuts against the inner side of the mechanical pump drive gear and is securely connected to the mechanical pump drive gear. Another end of the distance ring is spaced from the one end of the sun gear shaft in the spline shaft.