Abstract: In laboratory environments, the finite dimensions of water tanks (5.50 × 3.50 × 3.50 m) limit the ability to fully eliminate acoustic reflections from the tank walls. Particularly in low-frequency ranges, superposition of reflected sound fields tends to occur, significantly compromising the calibration accuracy of hydrophone sensitivity. By integrating Time Delay Spectrometry (TDS) into the free-field comparison calibration method—and employing a tracking band-pass filter to suppress reflected signals superimposed on the direct wave—the desired direct-wave signal can be effectively extracted, thereby extending the lower frequency limit of sensitivity calibration. To overcome the limitation of conventional TDS, which requires spectrum analyzers equipped with center-frequency-tuned filters as measurement instruments, this study proposes a software-based implementation that uses algorithmic processing and data acquisition systems to emulate hardware-like tracking band-pass filtering and real-time data processing. Experimental verification confirms the feasibility of software-implemented TDS for hydrophone sensitivity calibration. Results show that, in an anechoic tank environment, the proposed technique effectively suppresses measurement errors induced by wall-reflected sound fields—outperforming traditional pulsed-sound methods—and achieves hydrophone sensitivity calibration over the 1–12 kHz frequency range with an expanded uncertainty of 1.3 dB.