To accurately obtain the waveform and spectral characteristics of saline soil subgrade, on the basis of the provincial highway widening project of Ruoqiang-Weili county in Sinkiang, the field detection of coarse-grained weak sulfuric acid saline soil subgrade was carried out by using the type LTD—2100 ground-penetrating radar (GPR) to obtain the original measured files under different water content ($\omega$) levels. The IDSP analysis program was used to process the measured file and obtain the initial characteristics of waveform and frequency spectrum. To verify the initial characteristics, a rectangular model box was created in the laboratory with the field packing (the size was $1\times 0.8\times 0.8m$), and the water content of the model packing and temperature were manually adjusted. The model property of the waveform and frequency spectrum was obtained by detecting the model. A comparison of the initial property with the model property shows that under a positive temperature, the increase in $\omega$ will enhance the electromagnetic reflection, and when $8\%<\omega <27\%$, the reflection amplitude reaches the maximum, the maximum normalized amplitude is close to 1.0, the spectrum energy is dispersed, and the main frequency is not critical while it is lower than 200 MHz. When $\omega >32\%$, the electromagnetic loss is significantly intensified as the line mapping shows typical snowflakelike characteristics, and the point mapping shows typical linear characteristics, the spectrum energy is concentrated, the main frequency distribution range is 20–65 MHz, and the low frequency characteristics are evident. The results can provide a relevant reference for GPR detection and interpretation of coarse-grained weak sulfuric acid saline soil subgrade.