In the process of generating terahertz(THz) waves using femtosecond lasers, the plasma absorbs the THz wave energy, and the absorption characteristics have a great application prospects in the THz wave radar detection, plasma stealth, electromagnetic interference research. A measurement system of THz absorption via plasma was designed combining the theoretical analysis, and it was proposed that the interaction between plasma particles was the main reason for the plasma absorption of terahertz waves. The results show that the plasma density, the focal length of the optical lens and the angle of the incident femtosecond laser and the multiplier crystal are the main factors that affect the absorption degree, which provides a more comprehensive theoretical support for the plasma absorption application, it will be helpful to promote the THz wave technology in the military and civilian areas with rapid development.