A method for the determination of environmental persistent free radicals (EPFRs) in soil by electron paramagnetic/spin resonance spectroscopy (EPR/ESR) was established. In this paper, the central magnetic field, microwave power and modulation amplitude of the EPR spectrometer (x-band continuous wave) are tested and optimized. Based on the measured microwave frequency ν (9.82 GHz) and the g factor of the EPFRs, the central magnetic field of the spectrometer can be set to about 3 502 G. The microwave power saturation curve method and the microwave attenuation comparison method are used to optimize the microwave power. The two methods have their advantages and disadvantages, should be applied complementarily by both. Compared with 27 dB, microwave attenuation is set to ≤15 dB, and the peak (g=2.003 0 ~ 2.003 5) is suppressed by more or less due to power saturation. Therefore, about 27 dB or 25-30 dB is considered to be the optimal microwave attenuation value of this peak. However, the response of the other peak of higher magnetic field (g=2.000 4) is different, and the peak intensity is stronger at the higher attenuation value. In addition, the modulation amplitude should be smaller than the least peak-peak width (3.28 G), and 1 G is suitable if the narrower peak (1 G) is also studied. This method can be applied to the determination of EPFRs in soils and environmental solid samples, and it can ensure the accuracy of g factor, the resolution and the reliability of superfine splitting of EPR spectra.