Determining the location of the soil wetting front is of great significance for optimizing irrigation decision-making and realizing intelligent irrigation. The automatic device for detecting the soil wet-dry interface has not been commercialized at present. A soil wetting front measurement system was proposed with a vector network analyzer based on the research of TDR impedance measurement. Through TDR waveform reconstruction, the shortcomings of traditional TDR systems were effectively solved, such as insufficient bandwidth, long duration of rising edge of pulse and unclear boundary position detection. The reliability of the TDR waveform reconstruction method model was verified by simulation with ADS software. The locations of oil-water interface, dry-wet interface of two layers of soil with different moisture contents, and dry-wet interface of three layers of soil with different moisture contents were measured. The comparison between the results obtained by using the built model and the results obtained by the TDR method demonstrated the accuracy of the measurement. The theoretical simulation results carried out by ADS software showed that the TDR waveform converted from high frequency domain to time domain can accurately measure the position of impedance discontinuities. Through the two-layer and three-layer soil measurement experiments with different water contents, it was shown that this test method can be applied to actual production measurement. The accuracy of the reflection signal conversion model based on potable vector network analyzer in the actual application of determining the location of the soil moisture front provided a theoretical reference for the determination of the distributed soil moisture content.