The invention relates to a method for preparing kaolin used by glass fiber. The method for preparing kaolin used by glass fiber is characterized by comprising following steps: 1) pre-calcining raw kaolin ore power at a temperature of 600 to 650 DEG C for 2.0 to 2.5 h so as to obtain the pre-calcined kaolin; the particle size of the raw kaolin ore power is smaller than 325 meshes; 2) adding an additive to the pre-calcined kaolin, wherein the additive is ZnO and CeO2, the addition of the ZnO accounts for 1 millesimal to 5 millesimals with respect to the mass of the pre-calcined kaolin, and the addition of the CeO2 also accounts for 1 to 5 parts per thousand with respect to the mass of the pre-calcined kaolin; blending and mixing so as to obtain the kaolin used by glass fiber; the content of water absorbed in the obtained kaolin used by glass fiber is H2O- less than or equal to 2.0 percent. The invention eliminates the phenomena of foams, coloring and infusible substances which occur when domestic kaolin is used as a filler to by applied in the glass fiber industry, and meanwhile is more favorable for the stability of the homogenization and quality of the raw glass fiber ore; the prepared kaolin conforms to the national standards and industry requirement, can replace the imported kaolin used by glass fiber and meanwhile reduces the production cost of the glass fiber.