Grounding grid is an important part to maintain the safety and reliability of power system, thus the accurate assessment of the grounding state is related to the safe and stable operation of power system directly. In view of the problem that the security classification is only based on experience and considering whether the single characteristic parameter satisfies the limit, this paper proposes a comprehensive evaluation method based on dynamic trend (DT) method and rough set theory. The data are classified reasonably using the DT method. According to the acquired safety limit value from the guidelines and procedures, the discrete processing of the measured data are conducted. Besides, the best comprehensive weight of each characteristic parameter is calculated by the application of rough set theory. Finally, the comprehensive assessment results of grounding grid state are gained with the linear weighted method. The rationality and validity of the proposed evaluation method is demonstrated by the results of application on an engineering example. This work is supported by Research and Development Plan of Science and Technology in China Railway Corporation (No. 2013J010-E) and Natural Science Foundation of Gansu Province (No. 148RJZA053).