The invention provides an online stability evaluation index system of an electric power system. The online stability evaluation index system comprises an online data integration module, a power grid operating state index classification and evaluation module, a power grid variation trend index customization and calculation module, a power grid variation trend index display module and a power grid variation trend decision-making aid module. On the basis of an online integration method of basic data, index classification and calculation are carried out accompanied by index display, and thus a reliable and reasonable basis is provided for aiding decision making of future trend variation of the power grid. Algorithm development, induction, summarization and classification are carried out on indexes reflecting the future operating state of the power grid in all aspects, and the future operating state of the power grid is clearly displayed. What needs to be protected mainly includes the design framework and the function content of an index system, an index classification method and a method adopted for a platform index customization tool and a formula parser.