In this study, a detection method for trace Hg in soil samples using a sample preparation process that included air drying, grinding, sieving, mixing, and compaction, combined with high-precision X-ray fluorescence (XRF) spectrometry using monochromatic excitation was established. Calibration curves for mercury were developed using 11 certified reference materials from soils and sediments, achieving a linear correlation coefficient of 0.997 6. The detection limits for Hg in soil were determined to be 0.05 mg/kg using the background method and 0.2 mg/kg using the repeated measurement method. Following the quality control standards outlined in HJ780—2015, a comparison between high-precision XRF measurements and laboratory methods was conducted on 29 different soil samples, yielding a correlation coefficient of 0.995. Notably, for samples exceeding 0.5 mg/kg of Hg content, the accuracy pass rate was over 90%. Compared to laboratory methods, the monochromatic excitation high-precision XRF technique allows for rapid and precise on-site detection of trace Hg with only air drying, grinding, and sieving required, making it a simple, fast, and accurate field testing method.