For the isolated bidirectional triple active bridge (TAB) DC-DC converter, high reactive power and low efficiency happens when the voltage of each port is mismatched. To solve this problem, this paper proposes an efficiency optimization strategy based on minimizing reactive power. First of all, the unified mathematical expressions of voltage and transmission power of TAB converter are established based on Fourier series. Second, through the analysis of the physical meaning of the vector diagram, the constraints that must be satisfied to obtain the minimum reactive power are derived. Based on this, the obtained constraints are solved globally by the particle swarm optimization(PSO) algorithm, which avoids the complex calculation process introduced by piecewise linearization. The simulation results proves that the optimization strategy can significantly reduce the backflow power and the root-mean-square value of the inductor current under port voltage mismatch, and the system efficiency is improved.