For the vehicle-mounted soil conductivity measurement system based on the current-voltage four-terminal method, the excitation source will fluctuate with the change of soil load, thereby affecting the measurement results. Using waveform standard and strong anti-interference ability adjustable frequency and amplitude modulation digital signal generator instead of ordinary excitation source, by exploring the influence of different frequencies and amplitudes of the signal generator on the experimental results, and then the most suitable frequency and amplitude were fond for the vehicle-mounted measurement system value. In order to reduce the impact of complex environment of field on the experimental results, the experiment was divided into laboratory exploratory experiments and field confirmatory experiments. Laboratory exploratory experiments set five amplitudes and 20 frequencies for experiments. By analyzing 6000 sets of laboratory data of 60 sets of samples, it can be known that the larger the amplitude at the same frequency was, the better the experimental effect was, and the best amplitude was 10V. Under the same amplitude, with the increase of frequency, the soil conductivity showed a parabolic trend which was increased first and then decreased. The measured value of the soil conductivity and the reference value determination coefficient R 2 reached 0.9505 at the amplitude of 10V and frequency of 100Hz, and the experimental effect was the best. The best amplitude of the field confirmatory experiment was still 10V, but because the soil load was increased with the increase of the electrode spacing, the best frequency of the field confirmatory experiment became 1kHz, with an amplitude of 10V and a frequency of 1kHz. The measured value of conductivity and the reference value R 2 were 0.8484, which was the best experimental effect.