The aim of this paper is to provide a quantitative stability analysis of a time-delay power system stabilizer（PSS） system with centralized wide-area control using additional global signals. Frequency-sweeping（FS） test is proposed to compute the delay margin of two-level PSS design and the system with additional inter-area damping， and the control performance is further estimated quantitatively. Simulation results verify the validity and pragmatism of the proposed method.