An advanced criterion of optimal sensor placement was proposed for aeroservoelastic analysis of high-aspect-ratio flexible aircraft based on classical sensor placement index. The trace of observability Gramian matrix was proved to be the vibration energy, and the dimension of the ellipsoid was introduced as the geometric mean of eigenvalues, a high efficient simplified method was given as a computational approach. Different modes and sensors placement influences on the system were evaluated though the properties of H₂ norm in connection with system impulse response, a balance between the low and the high frequency modes was achieved to guarantee the dominant of low order vibration and avoid overflow. Considering the bending moment of the body, the bend and torsion of the wings, the different sensor placement indexes were evaluated. Results shows that the sensor placement can be set accurately and effectively, while different modes can also be fully weighted and reflected by the proposed criterion. This method can be used as an optimal tool to set the sensor placement through the whole flight track and it has a great value in engineering application.