In this work, hot-filament chemical vapor deposition (HFCVD) was employed to fabricate four groups of microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) films with continuously tunable grain sizes on silicon substrates by adjusting the volume fraction of methane (CH₄) in the gas phase from 1% to 5%. Based on terahertz time-domain spectroscopy (THz-TDS), the terahertz optoelectronic properties of the two types of diamond films were systematically characterized at room temperature within the frequency range of 0.2-1.2 THz. A vertical reflection measurement configuration of THz-TDS was adopted for the experiments, and the test principle is displayed in Figure a. The variation laws of terahertz reflected electric field amplitude and phase angle of the two film samples with radiation frequency were obtained through experimental tests. Taking the MCD film sample as the typical research object, its frequency-domain response curves are shown in Figure b, and the inset of Figure b presents the corresponding terahertz time-domain spectrum of the sample. The complex photoconductivity σ(ω) of the films was further extracted from the vertical reflection time-domain spectral data. Data analysis indicates that the classic pure Drude model fails to effectively fit the measured photoconductivity spectra. In contrast, the Drude-Lorentz model, which incorporates the scattering effect of free carriers and the resonance effect of bound electrons, achieves a good fitting consistency with the experimental data (Figure c). Key electrical parameters, including the ratio of carrier density to effective carrier mass R=N/m*, the electronic relaxation time τ, and the Lorentz frequency ω₀, were acquired based on the fitting results. Combined with the quantitative characterization results of X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy, the evolution laws of the microstructures and optoelectronic properties of the films were analyzed. With the gradual increase in CH₄ concentration, the grain size of diamond decreases continuously from approximately 2.1 μm to 83.9 nm, accompanied by a continuous increase in the grain boundary density of the films. Meanwhile, sp² graphitic carbon is gradually enriched at the grain boundaries, and the integral area ratio of sp²/sp³ phases increases monotonically from 0.32 to 8.31. The high-density graphitic carbon at grain boundaries acts as an intense carrier scattering center, which significantly reduces the carrier relaxation time and introduces localized bound electrons to induce terahertz resonance absorption. These effects ultimately lead to distinct differences in the optoelectronic properties between MCD and NCD films. Experimental tests and analytical results demonstrate that MCD films with large grain sizes possess lower grain boundary scattering loss and superior carrier transport performance, showing promising application prospects in diamond-based microelectronic devices. In comparison, NCD films exhibit weaker terahertz optical absorption intensity and have outstanding application potential as high-performance terahertz optical window materials. The research findings provide reliable theoretical and experimental support for the scientific research and industrial application of micro- and nano-crystalline diamond thin films in electronic and optoelectronic devices.