In view of the effect of time-delay to the system operation in the Wide Area Power System (WAPS), the stability of WAPS with time-delay is studied. Firstly, the new functional is constructed based on the Lyapunov stability theory. Secondly, this paper splits the integral into two integrals to avoid the scaling error and uses Wirtinger inequality to reduce conservative in the derivation, then considers the integration impact of the system state. The stability criterion with less conservative for time-delay power system is obtained and extended to the system with parameter changes. Finally, the effectiveness of the method is verified by simulation of the typical second-order system, the single machine system and two-area four-generator power system. This work is supported by National Natural Science Foundation of China (No. 61573130 and No. 61104119) and Program for Science & Technology Innovation Talents in Universities of Henan Province (No. 13HASTIT044).