Since Taylor series method introduced for power system transient stability analysis， it always has a good reputation of high calculating efficiency. But its computational efficiency is not maximized because it simulates the non-equivalent N -machine system with a large number of differential equations. To reduce the number of differential equations， this paper derivates a multi-group mode Taylor series method for transient stability analysis and gives a novel Taylor series expression of electromagnetic power， power angle based on equivalent parameters. This succinct and recursive expression has a better computational efficiency. Finally， the standard New England 10-generator power system is used to test the effectiveness of the proposed multi-group mode Taylor series method. The results show that the multi-group mode Taylor series method of equivalent parameters not only maintains the analysis accuracy， but also greatly improves the computational efficiency of the transient stability analysis because of the reduced number of differential equations used.