The state estimation in a multi-area interconnected power system is investigated and a new approach based on sensitivity analysis is presented. The state estimators of subsystems at both ends of tie lines carry out individual calculation. And the mismatch power obtained is taken as the virtual variation of measurement. Based on the relations between the sensitivity among state variables， measurements and power estimates， the state vector and power estimates of the interconnected power system are resolved in the computation center by iteration. Test results show that the calculation is very accurate and rapid. Even with part of the subsystem data unavailable， this approach can still provide the state estimation solution for the interconnected power system composed of the observable subsystems.