Abstract: The influence of the nonlinear interaction between femtosecond plasmas on the ultra-high frequency electromagnetic wave—terahertz (THz) wave was mainly studied in this paper. Many research institutions had confirmed that in the process of generating THz wave, the interaction between plasmas would affect THz wave. Combined theoretical analysis, a test system for generating THz wave due to the coincidence of two-beam plasmas was designed by the author. The study found that the nonlinear interaction between plasmas will produce a third-order nonlinear optical effect. The refractive index and phase of the plasma changed to form a non-uniform field, which led to a decrease in the radiant energy of the THz wave. In the experimental measurement research, it was found that with the increase of the plasma wavelength of the double beam plasma wavelength, the plasma density increased, resulting in a more obvious decrease in the radiation energy of the THz wave. In addition, the greater the plasma power, the greater the absorption of THz wave. At the same time, it was found that the mass of inert gas molecules around the plasma affected terahertz reduction degree. The mass of gas molecules determined the intensity of the electric field formed by the plasma ionized by the femtosecond laser focusing on the air. The higher the molecular mass, the stronger the intensity of the electric field formed, and the greater the magnitude of the THz wave reduced when the two-beam plasma overlapped. The study provides more comprehensive theoretical support for influence of the nonlinear interaction between plasmas on the THz wave, which helps to promote the rapid development of THz wave technology in the military and civilian fields.