A method to determine the time-delay upper-bound of the existing wide-area controller of low frequency oscillation based on improved free-weighting matrix and generalized eigenvalue problem (GEVP) is proposed. Firstly, a new class of Lyapunov-Krasovskii functional is constructed. Secondly, the derivation function along the system is solved. On this basis, necessary loose items are added to the derivation function in order to reduce the conservative, and a set of matrix linear inequalities is formed. Then the upper-bound of delay stability of wide-area controller is solved by means of GEVP. Finally, the time-domain simulation of the IEEE 4-machine 11-bus test system and IEEE 16-machine 68-bus test system demonstrate that the method of solving time-delay stability upper-bound of power system wide-area controller based on improved free-weighting matrices can be applied to different control methods and different types of power systems.