The vibrational and rotational energy levels of many biomolecules are in the terahertz band, so terahertz time-domain spectroscopy can be used to detect biomolecules. In addition, the photon energy of terahertz wave is low, only in the order of MeV, it will not damage biological samples in the detection process, so terahertz time-domain spectroscopy has an extensive application prospect in the research fields of biochemical detection in the future. Studies have shown that most biomolecules need to be in a liquid environment to give full play to their biological activity. However, the hydrogen bond in an aqueous solution will produce strong absorption in the terahertz band. What’s more, water molecules are polar molecules, and terahertz wave has strong resonance absorption to polar molecules, which makes it very difficult to detect active biomolecules in a liquid environment by terahertz technology. Therefore, many research teams combine terahertz spectroscopy with microfluidic technology to reduce the impact of various factors on biomolecular detection. Microfluidic technology reduces the distance between liquid sample and terahertz wave by reducing the depth of liquid pool in microfluidic chip, reducing the absorption of terahertz wave by aqueous solution. In this study, a double-layer microfluidic chip was prepared by cycloolefin copolymer (COC: Zeonor 1420R), and its transmittance to terahertz wave was as high as 95%. The length and width of the liquid pool in the microfluidic chip are 4 cm, and the depth is 50 μm. Furthermore, there are a large number of free-moving cations and anions in the electrolyte. Therefore, we used an external electric field device to apply voltage to the microfluidic chip injected with liquid samples to explore the influence of free-moving cations and anions in the electrolyte on terahertz transmission characteristics. The external electric field device includes a power supply, a ZVS circuit encapsulated in a plexiglass box and a DC high voltage package with an output voltage of 10 000 V.On this basis, we studied the terahertz wave transmission characteristics of five potassium salt solutions with the same concentration and these five potassium salt solutions in a constant electric field for different times, which provides a basis for further enhancing the application of terahertz time-domain spectroscopy in biochemistry. Moreover, the electrolyte contains many positive and negative ions, which will move under the action of the external electric field. And it provides technical support for terahertz time-domain spectroscopy to study the dynamic characteristics of electrolytes.