Abstract: The delay stability criterion, based on the Lyapunov stability theory and linear matrix inequality（ LMI）, is widely used in stability analysis of the power system with time delays. However, it is hardly applicable to high dimensional systems where time consumption is high and computation efficiency low. The number of variables to be calculated increases by a factor of square as the dimension of the state variable increases. For this reason, a numerical calculating method is used to study the time-varying properties of each item of Lyapunov functions to obtain the weight of each item. Then a delay stability criteria improved method is proposed, in which redundant items of the functions are identified and deleted to improve the efficiency of the stability criterion without increasing its conservatism. Finally, the delay stability criteria improved method is used for two criteria in a typical second-order-system and a 3-machine 9-bus system with two time delays to verify the efficiency of the proposed method.