The disadvantages of traditional small signal stability analysis method are analyzed for interconnected large-scale power system， and the combined analysis method based on WAMS and singular entropy matrix pencil is proposed. Singular entropy is applied to determine the mode order in the proposed method. It can determine mode order accurately regardless of the existence of noises. It also can reduce the computational complexity. Taking the power fluctuation incident which was resulted from switch explosion on June 17th 2009 in the Central China Grid as an example， the results of conventional small signal stability analysis method and the proposed method are compared. The simulation results show that the proposed method is a potential supplement for the traditional low frequency oscillation analysis method， and can reflect the oscillation characteristics with higher precision in timely and more comprehensively. It is an effective method for studying low-frequency oscillation resulting from multiple disturbances or some special operation mode.