The assessment of stability margin includes two tasks: the acquisition of disturbed trajectories and analysis of trajectory energy. Disturbed trajectories acquired by phasor measurement unit（PMU） can avoid the unfaithfulness of models， parameters and disturbance scenarios adopted for simulations. To quantify the stability of the disturbed system exactly， the calculation of the trajectory stability margin should not use supposed models. In a model-free situation， the stability of unstable trajectories can be quantified exactly based on extended equal-area criterion（EEAC） while the stability quantification of stable trajectories faces great difficulty. Online identification for a system of high dimensions and time-variation is even more difficult. Therefore， the extrapolation of power-angle curve is adopted to predict the imaginary trajectory after the farthest point（FEP） and potential energy. Time-variation indices are proposed to identify the cases with large error in extrapolation. The proposed method is validated on IEEE 39-bus system and East China power grid.