Terahertz air-based coherent detection technology is an important detection method in the field of terahertz. By using liquid water (or other liquids) as the interaction medium, this detection technology can be improved. The basic principle of this detection technology is introduced, and the effect of laser second harmonic and its intensity on the detection results are analyzed in detail. The characteristics and properties of the detection results in the time and frequency domains under different ratios of terahertz electric field and intensity signal at different intensities are analyzed. The evolution law of the technology from incoherence to coherence at different laser second harmonic intensities is analyzed, as well as the influence of the intensity of the probe and the nonlinear coefficient of medium on the detection. The application of solid-state materials in this technology is discussed. The effects of gas, liquid, and solid-state materials on the generation of second harmonics and detection results are compared in this technology. The research results are helpful for understanding and using liquid-based terahertz broadband coherent detection technology.