In the paper, SAP2000, a finite element software, is used to analyze the natural frequency and vibration shapes in order to investigate if the cable-stayed bridges will appear coupling from resonance under earthquake effects. Moreover, we study dynamic behaviors of cable-stayed bridges by using modal superposition method. We also do the comparison of the results from Complete Qudratic Combinatin Method and Square Root of Sum of Squares Method. Consequently, we use the acceleration spectrum of the China specification to analyze the examples in this thesis to discuss if the acceleration spectrum of Taiwan for cable-stayed bridge is too conservative.