In order to solve the design problem of coupled inductor in multiphase interleaving bidirectional DC/DC converter, the universal design criterion of coupled inductor with general applicability was proposed. The Buck+Boost interleaving and magnetically integrated bidirectional DC/DC converter was studied under Buck mode with all inductors coupled symmetrically. By analyzing the distribution of modes, the expressions of universal equivalent steady-state inductor and transient inductor in multiphase interleaving and magnetically integrated bidirectional DC/DC converter were obtained. The relationship among steady-state current ripple, transient current response speed, duty cycle, number of phases and coupling coefficient was deduced. The universal design criterion of coupled inductor in multiphase interleaving and magnetically integrated bidirectional DC/DC converter was summarized. When coupling coefficient and number of phases were chosen under the design criterion with constant duty cycle, the steady-state phase current ripple could be reduced and the transient total output current response speed could be improved at the same time. At last, the design criterion was verified by experimental results.