The invention relates to a method for mending a primary tree hole, which comprises the following steps: preparing raw materials; cleaning up the tree hole; sterilizing the tree hole; preparing a sizing agent: mixing and stirring dried wood flour, fiber or fine wood shavings with an inorganic adhesive in a mass ratio of 1-2:1 to prepare the sizing agent; filling the sizing agent: after an inner wall of the tree hole is dried until the water ratio is smaller than 14 percent, pouring the sizing agent into the tree hole, and sealing and fixing outside the hole opening until the sizing agent is filled in the gap in the hole to ensure that the sizing agent is filled and adhered to an inner side of the tree hole; curing the sizing agent; and performing simulation modification. Therefore, the sizing agent filled in the tree hole has natural characteristic index which is more approximate to the tree per se, and has toughness, corrosion resistance and fire resistance and enhances the effect of strengthening the tree. In addition, the technology is simple and the repair quality is reliable.