Indium antimonide (InSb) is a narrow-bandgap semiconductor that supports intrinsic surface plasmon resonance (SPR) effects in the terahertz (THz) frequency range. By avoiding the manufacturing complexities and low quality factors associated with artificial spoof surface plasmon polaritons (SSPPs), it provides favorable conditions for experimental investigations of THz-SPR-based biochemical sensing methods that have long been overlooked. In this work, the InSb grating-coupled THz-SPR mechanism and properties were first analyzed through numerical simulations using the Drude-Lorentz material model. Based on these theoretical results, which optimized the grating ridge height to 7 μm and the duty cycle to 0.5, periodic InSb gratings with varying periods ranging from 100 to 160 μm were designed and fabricated. The fabrication utilized standard semiconductor processes on a heavily Te-doped InSb wafer, including deep reactive ion etching (DRIE) and inductively coupled plasma (ICP) etching. Using a custom-built reflection-mode measurement module integrated with a THz time-domain spectrometer (THz-TDS), the p-polarized THz reflection spectra of the InSb gratings in dry air were measured at a fixed incident angle of 30°. High-resolution frequency spectra were acquired by applying zero-padding and fast Fourier transform (FFT) to the time-domain signals, revealing extremely sharp intrinsic THz-SPR absorption peaks. The experimentally measured relationship between the resonance frequency and the grating period matched the theoretical calculations excellently. Subsequently, the THz-SPR biochemical sensing performance of an optimized InSb grating with a period of 130 μm was systematically investigated. In the unloaded state, this sensing chip exhibited a distinct resonance frequency of 1.540 THz, a narrow full width at half maximum (FWHM) of 8.20 GHz, and a high quality factor of 188. By utilizing glycerol-ethanol mixed solutions with varying volume fractions as the dielectric media, a high refractive-index sensitivity of 0.665 THz/RIU for liquids was achieved at an incident angle of 30°, significantly outperforming most reported metasurface-based THz-SSPP sensors. Furthermore, to overcome the severe interference caused by the strong THz absorption of liquid water, a specific detection method based on a calcium carbonate deposition reaction was introduced. Through the selective combination and sedimentation of target ions using a saturated sodium carbonate solution, the quantitative detection of calcium ions in aqueous solutions was realized using the InSb grating THz-SPR sensing chip. The experimental results show that, within the concentration range of 1-10 mmol/L, the THz-SPR resonance frequency increases linearly with increasing calcium ion concentration, demonstrating a high sensitivity of 0.0032 THz/(mmol/L) and excellent measurement consistency. Good repeatability was also demonstrated for the detection of 1 mmol/L calcium ions by restoring the initial resonance frequency after cleaning. Furthermore, by exploiting the calcium carbonate deposition reaction on the surface of the InSb grating, the concentration of free calcium ions in fetal bovine serum samples was measured. The sensing chip detected an ionized calcium concentration of 2.08 mmol/L. Comparison with total calcium concentration results obtained by a third party using inductively coupled plasma mass spectrometry (ICP-MS) verified the reliability and validity of the proposed method, as the measured free calcium closely matches the standard proportion of ionized calcium in serum. The experimental results reported in this paper fill a research gap in the field of THz-SPR biochemical detection in China. By demonstrating a structurally simple, highly tunable, and semiconductor-process-compatible platform, this work highlights the unique advantages of intrinsic THz-SPR in achieving high-Q biomolecular detection, laying a robust experimental foundation for the on-chip integration and practical application of terahertz biochemical sensors.