The safety feature of the ultra-high voltage power grid brings the needs of the risk assessment method that is able to consider the transient angle stability. The existing transient angle stability evaluation methods employ large amount of calculation and thus can not meet the requirements of the risk assessment of the ultra-high voltage power grid. This paper studies the factors that influence the transient angle stability after system fault, and analytically derives the sensitivity of these factors to system critical stability based on Extended Equal-Area Criterion (EEAC) method. The method of approximately evaluating transient angle stability based on typical failure set after the approximately judging boundary condition changes is then proposed. Finally, the framework of power system risk assessment method is carried out considering transient angle stability. It provides new ideas to the risk assessment method of ultra-high voltage power grid.