An energy management system（EMS） planning context power analysis system is designed and developed. The system is integrated in EMS based on multiple-context mechanism， and can sufficiently utilize on-line operation data， either real-time data or historical data. Based on single-line diagram， users can dynamically modify and construct network model for future power system， and investigate power flow and static security. The paper introduces the framework of functional modules at first， then presents the design and development technique of subsystems including figure， modeling， algorithm and data accession. At last， the system implementation in an actual provincial power system in China is presented.