An assembly structure of sun gear shaft and spline shaft of a gearbox for wind turbine, comprising: a sun gear shaft; a spline shaft, wherein the spline shaft is a hollow shaft, one end of the sun gear shaft is inserted into the spline shaft, and the one end of the sun gear shaft inserted into the spline shaft is connected to the spline shaft through a spline pair; a mechanical pump drive gear, wherein the mechanical pump drive gear is mounted at one end of the spline shaft, and an inner diameter of the mechanical pump drive gear is less than an inner diameter of the spline shaft; and a distance ring, wherein the distance ring is disposed in the spline shaft, one end of the distance ring abuts against an inner side of the mechanical pump drive gear and is securely connected to the mechanical pump drive gear, and another end of the distance ring is spaced from the one end of the sun gear shaft in the spline shaft; wherein the distance ring includes a distance ring side wall parallel to the spline shaft and a distance ring bottom perpendicular to the spline shaft; the distance ring side wall abuts against the inner side of the mechanical pump drive gear and is securely connected to the mechanical pump drive gear; and the distance ring bottom is spaced from the end of the sun gear shaft in the spline shaft; characterized in that: wherein a first bolt hole is disposed at an end of the distance ring side wall connected to the mechanical pump drive gear, the first bolt hole is disposed along an axial direction of the distance ring, the mechanical pump drive gear is provided with a second bolt hole corresponding to the first bolt hole, and a first bolt is inserted into the second bolt hole and the first bolt hole so that the distance ring is secured to the mechanical pump drive gear. a sun gear shaft; a spline shaft, wherein the spline shaft is a hollow shaft, one end of the sun gear shaft is inserted into the spline shaft, and the one end of the sun gear shaft inserted into the spline shaft is connected to the spline shaft through a spline pair; a mechanical pump drive gear, wherein the mechanical pump drive gear is mounted at one end of the spline shaft, and an inner diameter of the mechanical pump drive gear is less than an inner diameter of the spline shaft; and a distance ring, wherein the distance ring is disposed in the spline shaft, one end of the distance ring abuts against an inner side of the mechanical pump drive gear and is securely connected to the mechanical pump drive gear, and another end of the distance ring is spaced from the one end of the sun gear shaft in the spline shaft; wherein the distance ring includes a distance ring side wall parallel to the spline shaft and a distance ring bottom perpendicular to the spline shaft; the distance ring side wall abuts against the inner side of the mechanical pump drive gear and is securely connected to the mechanical pump drive gear; and the distance ring bottom is spaced from the end of the sun gear shaft in the spline shaft; characterized in that: wherein a first bolt hole is disposed at an end of the distance ring side wall connected to the mechanical pump drive gear, the first bolt hole is disposed along an axial direction of the distance ring, the mechanical pump drive gear is provided with a second bolt hole corresponding to the first bolt hole, and a first bolt is inserted into the second bolt hole and the first bolt hole so that the distance ring is secured to the mechanical pump drive gear.