<p>Wireless power transmission technology (WPT) is an ideal way to realize power supply for underwater equipment, which has a wide application prospect. However, the eddy current effect caused by the high conductivity of seawater not only leads to power loss, but also affects the resonant frequency of WPT system. Based on the principle of magnetically coupled resonant wireless power transmission (MCR-WPT), a wireless power transmission circuit model suitable for seawater environment is proposed in this paper, and the law of resonant frequency of WPT system changing with transmission distance in seawater is systematically studied. By constructing a mathematical model, the mutual inductance of the coupling coil in seawater is analyzed and calculated, and the eddy current effect, distributed capacitance and their influence on the equivalent inductance of the coil are deeply discussed, which leads to a shift of the resonance frequency of the system. The validity of the theoretical analysis is verified by physical experiment, and the transmission efficiency and the power loss in seawater of the system are compared under the optimal working frequency and fixed theoretical resonance frequency. The experimental results show that the transmission efficiency of the system is improved by 6.8% at the optimal working frequency compared with the fixed theoretical resonant frequency, and the power transmission loss in seawater at the transmission distance of 15&#xa0;cm is significantly reduced by 15.2% under the optimal working frequency compared with the fixed theoretical resonant frequency. This study provides a theoretical basis for frequency optimization of WPT system in seawater environment and provides important guidance for dynamically adjusting working frequency in practical application to improve system efficiency.</p>