Due to the low acoustic radiation resistance in the ultra-low frequency band (below 100 Hz), the electroacoustic efficiency of ultra-low frequency sound sources is extremely low (typically ≤1%). When deployed on unmanned underwater vehicles (UUVs), their high energy demand severely limits the operational endurance of the platforms. This study proposes a mutual radiation enhancement method based on a multi-vibrator configuration and presents a high-efficiency electrodynamic transducer structure to enhance the electroacoustic efficiency of ultra-low frequency sound sources for UUVs. The acoustic field coupling effect produced through inter-vibrator mutual radiation enhances the radiation resistance of each vibrator by radially arranging multiple driving vibrators densely within an electrodynamic transducer. Both the radiated acoustic power and electroacoustic efficiency are improved by exploiting the strong mutual radiation effects among vibrators to increase radiation resistance. The study establishes an equivalent circuit model for the high-efficiency multi-vibrator electrodynamic transducer, targeting the ultra-low-frequency range below 100 Hz. Modeling the radiation surface of the driving vibrator as an unbaffled single-sided circular piston radiator, the mutual radiation impedance between driving vibrators is calculated, and the physical mechanism by which multi-vibrator mutual radiation enhances electroacoustic efficiency is elucidated. Additionally, based on equivalent circuit and mutual radiation impedance theory, the transmitting current response is calculated, and a comparative analysis is conducted on the radiated acoustic power and electroacoustic efficiency between the N-element vibrator and the single-vibrator electrodynamic transducer. A six-vibrator electrodynamic transducer prototype is fabricated, and its transmitting current response and electroacoustic efficiency are measured to validate the theoretical model and analytical results. Due to intense mutual radiation effects in the ultra-low frequency range, each vibrator within an N-element electrodynamic transducer achieves N times the radiation resistance compared to its isolated operation. The radiated acoustic power of the transducer can increase to N² times the original level, while the electroacoustic efficiency improves proportionally to N times the initial value. Experimental results of the prototypes demonstrate that the transmitting current response of the six-vibrator electrodynamic transducer is approximately 15 dB higher than that of a single-vibrator structure, while its electroacoustic efficiency is increased sixfold. These measurements are in good agreement with the theoretical analysis.