Provided is a surface modification method of calcite powder prepared from desulfurized gypsum and heat-treated at a high temperature. According to the surface modification method, hydrophilic calcite powder having polarity may be changed into hydrophobic non-polar calcite powder by safely adsorbing a surface modifier onto a surface of the calcite powder and coating the surface with non-polar fatty acid. Particularly, in the case in which calcite powder heat-treated at a high temperature is hydrophobized, a significantly excellent hydrophobic surface property may be implemented.