Integrating solar energy with fossil fuel is a cost-efficient way to use solar energy to generate electricity in the near and mid-terms. In a solar hybrid coal-fired power system, concentrated solar heat is used to replace the extracted steam from the coal-fired power plant to generate electricity. Previous researches proposed several system configurations where solar heat is used to substitute extracted steam with different temperature and pressure in the coal-fired power system. The contribution of solar heat to the work output for such hybrid power system has been pointed out. However, the inherent reason of solar-to-electricity increased by solar hybridization with fossil fuel has rarely been examined. In this paper, on the basis of the concept of energy level, the correlation of the solar-to-electricity efficiency with the reduced exergy destruction of solar hybridization process is fundamentally derived and their interaction is revealed. In addition, the thermodynamic performance of a typical 330 MW solar hybrid coal-fired power system is evaluated by using the derived equations. The result obtained in this study is expected to provide a basic principle for designing the solar hybrid coal-fired power system.