This paper presents a methodology for evaluating the safety of slopes threatened by strong earthquakes. Based on the theory of modal analysis, the proposed approach analyzes the seismic landslide spectrum, including the calculation of the landslide intrinsic frequency and the investigation of the resonance effect. The concept and the solution of the resonance index (RI) of the seismic landslide are also proposed. By using the VC++ computer language, corresponding dynamic finite element program, namely Analysis of Seismic Landslide Spectrum (ASLS), is developed. To verify the proposed method, three examples are presented: a fixed-end beam, a specially designed landslide vibration experiment, and the largest landslide triggered by the 2008 Wenchuan earthquake. The obtained numerical and experimental results show that the proposed method can effectively evaluate the safety of seismic landslides. It is suggested that the resonance phenomenon of landslides should be considered in the landslide treatment and the seismic design.