IoT (Internet of Things) for EPESs (Electric Power and Energy Systems) is a fast-developing network built by the State Grid to monitor and control the power transmission and transformation equipments. Several domestic protocols and stan-dards have been published to guide the communication of node devices, of which each sub-system follows a different one. These protocols are designed to be very dynamic, with variability not only in field length but also in structure. Parsing modules of these protocols are of low reusability and hard to maintain as they are developed separately. Furthermore, the different implementations by different manufacturers bring the problem of conformance. In this paper, a structure description based method is proposed to satisfy the demand of multi-protocol parsing for devices in IoT for EPESs. The method includes description of message structure and a parsing algorithm dealing with variability.