从电子在THz波电场中的运动方程出发，讨论了THz波在等离子体中的传播规律.从而可以得到由等离子体电子密度和碰撞频率决定的THz波的复折射率.该复折射率决定了THz波在等离子体中的传播，即THz脉冲相位和振幅变化.THz时域光谱系统可以测量THz波传播的相位和振幅，因此可以利用THz时域光谱来诊断等离子体密度和碰撞频率.由于受到等离子体色散关系的限制，该方法测量的等离子体密度有一定范围.在等离子体电子密度位于1012~1016/cm3之间内，该方法可以得到较好的应用.

Based on the electron motion equation, the propagation of terahertz (THz) wave in plasma is studied. The complex refractive index of THz wave is determined by the plasma electron density and collisions. It decides the propagation of the THz in plasma, which means the change of the THz wave phase and amplitude. THz time-domain spectroscopy system can measure the THz wave phase and amplitude, so it can be used to diagnose the electron density and collisions. Limited by the dispersion relation in plasma, there is a limitation for this method in the electron density measurement. The diagnosis works well for plasmas with density in the range 1012~1016/cm3.