We examined the underground magnetic field near the lightning channel with low-frequency magnetic sensor based on SHAndong Triggering Lightning Experiment (SHATLE). Two sensors were deployed, one at 1-m height above ground and another one at 2-m depth underground at a distance of 78 m from the lightning channel, and the magnetic pulses during the initial stage of triggered lightning were recorded. The experimental results show that the microsecond-scale magnetic pulses radiated by the upward lightning leader can be detected in the subsurface space and the magnetic signal is modified by the soil medium. Specifically, the amplitude at the depth of 2 m is attenuated typically more than 55%, and the attenuation decreases as the timescale of the magnetic pulse increases; meanwhile, the peak time of the underground magnetic pulse is delayed by about 0.6 μs, and the half-peak width of the magnetic pulse is increased by 0.2–0.8 μs (namely, by 20% to 32%). The results of Fourier analysis indicate that the component with relatively high frequency is subject to more attenuation than is the component with relatively low frequency. In addition, the simulation of magnetic field with the channel-base current by using the transmission line model is consistent with the measurement, indicating that the modification on the waveform characteristics of the lightning pulse measured underground could provide valuable information for retrieving the electromagnetic parameters of soil.