Based on 4 span continuous straight bridges, the demands of inelastic displacement and elastic displacement of the bridge structures under earthquakes were obtained from inelastic time history analysis and elastic response spectrum analysis, respectively. Then the ratio of the inelastic displacement demand to its corresponding elastic displacement demand was investigated with parameter statistical method. It is found that the inelastic displacement demand approximates its corresponding elastic displacement demand for the bridge structures with long vibration periods. And because the elastic displacement demand can be calculated by relatively simple elastic analysis method (such as elastic response spectrum analysis), it can approximately replace the bridge structure’s inelastic displacement demand under earthquake to simplify its displacement based seismic design.