Firstly, the relation between SDOF equal displacement rule and yielding extent of structure is analyzed. And then, nominal yielding displacement and nominal yielding curvature are defined based on the traits of general continuous bridges. Finally, SDOF equal displacement rule extends to MDOF by analyzing on different kinds of continuous bridges with different girders, piers and earthquake inputs. Results show that the displacement of inelastic time history method is close to that of elastic response spectrum method and equal displacement rule is applicable when the periods of structure’s main modes are larger than the corresponding lower limit period of SDOF equal displacement rule and the sum of main modes’ participation mass factors is more than 90%, thus elastic displacement can be used to evaluate the corresponding inelastic displacement. The application of MDOF equal displacement rule simplifies the calculation of displacement demands in the transverse direction of a large number of continuous bridges and provides the basement for the displacement-based seismic design of those bridges.