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Unhydrated aminopropyl triethoxysilane was used to modify calcined kaolin produced from coal-bearing kaolinite. The aim was to develop a simple and economic modification method without the need for solution immersion and subsequent wastewater treatment. The samples before and after modification were tested using multiple methods, such as XRD, MAS-NMR, FT-IR, XPS, and SEM. The study results indicate that this modification can effectively improve the surface property of calcined kaolin. The activity index increased to 76.7% when the modifier usage was 2%. With a further increase in the modifier content, the activity index decreased. Surface modification did not reduce the whiteness of calcined kaolin. Further, the modification reaction mechanism was elucidated. Based on the detailed analyses, it was found that the modification reaction took place between the NH₂ groups in the modifier molecules and Al^VI-OH in calcined kaolin, and hydrogen bonds were formed between the NH₂ groups and the active sites on the calcined kaolin surface.