The present invention relates to a method of preparing a vinyl chloride polymer having good thermal stability due to the restraint of dehydrochlorination by heat or ultraviolet rays, and a vinyl chloride polymer prepared thereby. In the preparation method, a modifier is introduced at the end of a polymerization process to impart the vinyl chloride polymer with high thermal stability without inducing modification. Therefore, the generation of dehydrochlorination due to heat or ultraviolet rays may be markedly restrained, thermal stability may be improved, and discoloration or the modification of physical properties may be prevented in the vinyl chloride polymer thus prepared.