多运行方式下的系统特征根将以一定的概率分布在某些区间内。正态假定下的均值、方差可用来描述这种分布特性。当使用电力系统稳定器（ＰＳＳ）改善系统阻尼时，主要应考虑ＰＳＳ对临界特征根的均值、方差的影响。文中将传统的特征根灵敏度分析方法扩展为对特征根均值的灵敏度和方差的灵敏度。由此进行的ＰＳＳ选址和参数设计，将使ＰＳＳ具有多运行方式下的整体合理性。 The root of the system under multi-run mode will be distributed in certain intervals with a certain probability. The mean value under normal assumption, variance can be used to describe this distribution. When using power system stabilizer (PSS) to improve system damping, the influence of PSS on mean and variance of critical eigenvalues ​​should be considered. In this paper, the traditional method of eigenvalue sensitivity analysis is extended to the sensitivity and variance sensitivity of eigenvalues. The resulting PSS site selection and parameter design will make the overall rationality of PSS under multi-operation mode.