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At present, we often use critical clearing time and stability margin as the evaluation indices of the transient stability. We can calculate these indices by the SIME. But these calculations rely on simulation of unstable cases. However, in daily operations, power systems hardly fall into the unstable state and we operate in the stable case. We can operate the power systems more stably and effectively if we know stability margin in the stable power systems. In this paper, we propose a method using the deceleration area of the power-angle representation for transient stability assessment in the stable case. The SIME can assess transient stability of multi-machine power systems and detailed modeling fast. However, the SIME could not well handle simulations of stable cases, while the proposed method is able to expand the advantages of the SIME in simulations of stable cases. To investigate the validity of the proposed method, we show examples of 3-machine 9-bus systems.