In order to meet the requirements of global coordinated control and online security assessment of China Southern Power Grid（CSG）， the design of online distributed modeling system based on IEC 61970 standard for CSG is proposed. Models and graphs built in existed provincial energy management systems（EMSs） are transferred automatically to the control center of CSG through a wide-area power dispatch network， and then combined automatically into a consistent model for the whole 500 kV/220 kV AC/DC hybrid CSG. Some key technologies， including the determination and maintenance of model boundary， criterion for the re-denomination of equipment， extension of common information model（CIM）， combination of CIM， combination and organization of scalable vector graphics（SVG） based graph， online advanced applications for whole AC/DC hybrid CSG， mechanisms of model validation and operational maintenance， are introduced. Several performance indices are designed. Implementation of the designed system will provide whole online model for some developing application systems in CSG， such as new EMS， coordinated automatic voltage control， online security early warning and decision-making systems， etc.