Modern power system has been gradually developed into a complex system with the features of multi-level structure, multi-time scale, a variety of control parameters, wide-area, open, uncertainties, non-autonomous and it is difficult to indicate the security level of power system by a single index. Therefore, power system security assessment based on theory of comprehensive assessment is developed rapidly in recent years. However, traditional comprehensive assessment method considers few about the balance of the indices, when there are great differences between the value of the indices, it is easy to take place the phenomenon that the small indices has been “submerged”, which will influence the rationality of the comprehensive evaluation result. In this paper, a comprehensive assessment model which considers the influence of the index balance is established. By introducing the balance coefficient, the model can amend the original judgment matrix and the weight obtained by traditional AHP method. Since the coordinated and balanced situation of the indices have been reflected in the evaluation process, the results of the assessment model proposed in this paper has more scientific nature and more credibility. Finally, an example is provided to validate the model in this paper.