Portable near-surface frequency-domain electromagnetic (FDEM) instruments are effective tools for detecting underground electrical abnormal targets. However, due to the inductive reactance of the transmitting coil, conventional instruments cannot transmit high amplitude current at multiple frequencies at the same time in the high frequency range (>1000 Hz). This would lead to low detection efficiency and poor signal-to-noise ratio of the system and would ultimately affect the detection accuracy. In view of the above problems, a multi-frequency resonant strategy is proposed to simultaneously increase the transmitting current amplitude at multiple target frequencies. First, the multi-frequency resonance theory is analyzed and the circuit parameter solving method is improved. The validity of the multi-frequency resonance theory and the solution method is verified via circuit simulation. Further, the influence of the component parameter deviation on the circuit resonance is analyzed, and the selection principle of the resonance parameters based on the quality factor Q is presented. Considering the actual deviation, a five-frequency or seven-frequency resonant circuit may achieve a better resonance effect than the nine-frequency resonant circuit in practice. Finally, the strategy is validated with an actual circuit. The results show that the constructed five-frequency resonant circuit can effectively increase the amplitude at the target frequency. The maximum increase is 10.50 times, and the transmitting energy is 11.4 times higher. This study provides a feasible implementation method for multi-frequency resonance at five frequencies and above, which is helpful for improving the detection efficiency and signal-to-noise ratio of a portable near-surface FDEM system.