Information security risk assessment combined with the power control process remains an unresolved problem， the lack of an asset identifying method in conformity with the characteristics of power information system being one of the main factors. Network-based substation automation systems（SAS） are typical power information systems. Based on an analysis of the difficulties in asset identification， a complete information asset identifying method is proposed from three perspectives， including asset classification， business identification， and asset valuation. In asset classification， by referring to the IEC 61850 standard， a structured asset model is defined based on the association between actual intelligent electronic devices and virtual logical nodes. In business identification， with the functions of substation automation as the object of business， the concept of function graph is defined to reflect the logical relations between functions and assets， as well as the logical relation between the assets. In asset valuation， two levels of asset value including the function level and substation level， are defined to quantify the information security properties of assets and the effects of possible accidents. Finally， an evaluation instance is constructed according to the IEC 61850 standard for verification of the applicability of the method proposed to asset identification of network-based SAS.