Numerical integration method is the basic method of power system transient stability analysis and calculation. Looking for a fast and reliable numerical method can effectively improve the calculation speed and accuracy of power system transient stability. This paper puts forward a numerical integration method of power system transient stability which is based on Padé approximation, analyzes the numerical stability of this method and uses transient IEEE145-node calculation system for simulation calculation. By comparing the results of Padé approximation method with the results of Taylor series expansion method, it proves that the numerical integration method based on Padé approximation has high accuracy. Therefore the new method could be more suitable to numerical analysis of transient stability and other like-wise problems. This work is supported by National Natural Science Foundation of China (No. 51377098).