In order to obtain the ductility seismic performance of reinforced concrete box piers, equivalent plastic hinge length and ultimate displacement analysis method for reinforced concrete box piers were discussed based on bi-axial quasi-static test on 14 piers and moment curvature analysis of reinforced concrete box cross-section in action of bi-axial load. Results show: the piers with larger height-width ratio have bigger plastic damage area, meanwhile ultimate curvature declines obviously. Calculating yield displacement and ultimate displacement are close to testing ones, so the ultimate displacement calculation method could be used in ductility seismic analysis for the reinforced concrete box piers.