Abstract: In present, on-line structural damage detection attracts much focuses in the field of civil engineering. Extended Kalman estimator and optimal function were adopted for estimating the structural parameters and tracking the abrupt degradation of structural stiffness based on the partial accelerations, and the unknown external loading can be calculated in the same time through least squares estimation. Numerical example indicated that  the proposed algorithm can estimate the system parameters and the unknown external loading precisely, and track the exact moment, locations and extents of structural stiffness degradation on-line effectively using the partial observations of structural acceleration responses polluted by measurement noise