This paper characterizes the trajectory of severely disturbed power systems based on stability region theory， which will pass by controlling UEP（CUEP） on the verge of losing stability. Therefore the dynamic behavior of system near CUEP can be approximated by linear system to which local modal controllability and observability can be attached. PBH rank test will rule whether input or output matrix is able to control or detect unstable modes. Then eigenanalysis is performed on the linearized system. Eigenvalues， participation factors， and mode shapes can reveal the unstable mode of disturbance. And the mode controllability matrix and mode observability matrix can identify the most sensitive sites of control input and observation signal for the unstable mode. As an illustration， the proposed approach is applied to 10-machine 39-bus New England power system. The results demonstrate that the linear analysis approach based on CUEP lays a sound basis for the architecture of wide area monitoring and control.