Power system parameter variations have great influences on system stability and it is important to know which parameter variations are more decidable to system stability. An energy margin sensitivity calculation of power system is presented in this paper. The calculation method

based on mathematics expression of energy margin

uses full differential equation to calculate the sensitivity of energy margin

considers the influence of generator dynamic motion on trajectory sensitivity

and calculates the sensitivity of energy margin on system parameters. This method has been tested in IEEE 50-machine system and achieves good result.