Here, we proposed the thin-film total internal reflection geometry (TF-TIR) for sensitive material characterization. Equations to extract the material dielectric constant in the TF-TIR geometry was derived. The TF-TIR technique consumes less sample material and provides higher sensitivity compared with the traditional attenuated total reflection (ATR) geometry. The sensitivity of TF-TIR geometry was first investigated by simulation using a 10 μm thick α-lactose thin film as the sample. A THz microfluidic device was fabricated according to the TF-TIR design in the simulation with TOPAS and high-resistivity Si as the top and bottom plate, respectively. The reaction chamber was sandwiched between the TOPAS and Si plates. The device was placed on a right-angle Si prism to realize the total internal reflection. Water and alcohol mixtures were used to verify the sensitivity of the device. Our results demonstrate that the TF-TIR technique has the potential to improve the sensitivity in measuring the dielectric constant of biological samples with THz waves. Our design can be used for THz lab-on-chip devices.