Embodiments of the invention provide an electric power system transient stability assessment method and device. The method comprises the following steps of: obtaining a feature vector of a to-be-assessed point of an electric power system according to a pre-screened key state variable; respectively calculating a mahalanobis distance between the feature vector of the to-be-assessed point and a feature vector of each sample point in a sample set, and searching sample points adjacent to the to-be-assessed point according to the mahalanobis distances; and obtaining an assessment result of the to-be-assessed point according to a classification decision rule and the adjacent sample points. According to the method and device, the mahalanobis distances between the feature vector of the to-be-assessed point and the feature vectors of the sample points are calculated, and the mahalanobis distances sufficiently consider the relevancy between key state variables in the feature vectors, so that thedistance calculation is more correct, the adjacent sample points of the to-be-assessed points can be correctly searched, and the correctness of the assessment result is further improved.