When there are hidden troubles in power grid structure, it is easy to cause system out-of-step oscillation due to accidents. Impedance angle out-of-step separation criterion is the most widely used criterion. Its advantages are obvious, but there are also some shortcomings, that is, it is easy to misjudge in short-circuit transient process. In this paper, based on the in-depth study of the typical apparent impedance angle criterion theory, an improved criterion using maximum threshold of voltage envelope as unlocking condition is proposed. The scheme can strictly distinguish short circuit from out-of-step process, and improve the adaptability of apparent impedance angle out-of-step criterion to specific operating conditions and new power grid structure. Finally, the validity of the improved criterion is verified by a simplified example model in PSCAD/EMTDC. This work is supported by Science and Technology Project of State Grid Corporation of China “Framework Research and Design of Super Large-scale Power Grid System-level Control and Protection Technology” (No. SGBJDK00KJJS900088).