The three-dimensional orthogonal vibration direction converter is an ultrasonic vibration system integrating power synthesis and three-dimensional acoustic radiation, which has important application value in the field of power ultrasonic technology. At present, the analytical design theory of the three-dimensional vibration direction converter is mainly based on the complex wave equation method, and research on its vibration characteristics also focuses on the three-dimensional vibration direction converter with a symmetrical structure. Therefore, based on one-dimensional longitudinal vibration theory, the apparent elasticity method, and the electromechanical analogy principle, this paper establishes a concise equivalent circuit model for the three-dimensional coupled vibration of a three-dimensional orthogonal vibration direction converter. Several converters with different lateral and longitudinal dimensions are designed, and their vibration performance is analyzed by finite element simulation and experimental testing. The results show that when the transverse or longitudinal size of the converter in a certain direction decreases, the equivalent mass of the bar in this direction decreases, and the displacement amplitude of the output end in this direction increases under in-phase resonance. Conversely, increasing the size of the converter in a certain direction will increase the equivalent mass in that direction and reduce the displacement amplitude in that direction. It can be seen that by adjusting the transverse or longitudinal dimensions of the converter, the vibration displacement amplitude of each output end can be flexibly adjusted to meet the needs of diversified ultrasonic applications. The theoretical calculation results are consistent with the finite element simulation and experimental test results. The equivalent circuit model provides a concise design theory for the engineering design and optimization of the three-dimensional orthogonal vibration direction converter.