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The backflow power in dual-active bridge (DAB) converters can cause power circulation in the H-bridge instead of flowing to the load, which increases the power loss of switching devices and magnetic components and lowers the system’s efficiency. Hence, a unified optimal modulation strategy (UOMS) is proposed in this article to reduce the backflow power and consequently improve the system efficiency. However, there is a conflict between minimizing the backflow power and achieving the zero-voltage-switching (ZVS) conditions of all the switching devices. Hence, a regulatory factor “ $\gamma $ ” is introduced into the control variables to minimize the backflow power on the premise of all ZVS conditions, and hence, comprehensively improve the system efficiency. Moreover, the regulatory factor “ $\gamma $ ” simplifies the calculation form of the control variables, which is easier to perform in the controller and can effectively shorten the computational time. According to the value ranges of the control variables ( $D_{\mathbf {0}}$ , $D_{\mathbf {1}}$ , and $D_{\mathbf {2}}$ ) in the proposed UOMS, 12 operating modes are classified and four effective modes are detailed. For the four effective modes, the optimal triple-phase shift (TPS), optimal primary-dual-phase shift (PDPS), and optimal secondary-dual-phase shift (SDPS) modulation strategies are derived and their switching relationships are revealed. With various transferred powers, the proposed UOMS combines the optimal TPS, PDPS, SDPS, and SPS modulation strategies and can realize smooth and seamless transition switching processes among them in the full operating range. Therefore, the DAB converter can always operate in the optimal states for the full operating range of both the “Buck Mode” ( $k > $ 1) and “Boost Mode” ( $k < 1$ ). Finally, experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed UOMS.