The status of transmission network operaion is satified when the demand from the diverse transmission system stakeholders is satified. Therefore, the evaluation index system for transmission network operation is actually evaluating the demand satisfaction from the stakeholders in transmission system. Especially, the external demand is the demand for transmission network operation from the external stakeholders of the transmission system, including the generation enterprise, the distribution enterprise, the interconnected transmission enterprise. The external demand was analyzed and concluded as follow: the local transmission system should accomodate power energy with specified quality and quantity from or provide it to the specified node in the specified time. Different types of external nodes had different demands, so this paper studied different quality and quantity demands of the power energy from different external nodes in detail, and expounded the evaluation methods for the satisfaction degree of the external demand. According to the specific external demand, the scientific and reasonable index was selected to evaluate the instantaneous demand satisfaction degree or in statistics cycle time, thus the external demand index set was proposed for the evaluation index system of transmission network operation.