Study the calculation method of equivalent damping ratio in Capacity Spectrum Method. Analyzing the feature of several methods for the calculation of equivalent damping ratio in the world, from the numeral example, the numeral results of several calculation methods of equivalent damping ratio are compared to the results of time course method. The follow conclusions are reached: Secant stiffness method overestimate the value of equivalent damping ratio for steel structure, therefore, underestimate the supreme seismic reaction. The level of underestimate is interrelate to yield still coefficient; Under the medium earthquakes effect, all the method can fairly good forecast the supreme seismic reaction; But Under the large earthquakes effect, only the Kwan (EP) method can give the best result. The conclusions has impotent value for the perfect of Performance-Based Seismic Design Methods.