This paper has built the one-dimensional model of the time-varying un-magnetized plasma, and the finite different time domain (FDTD) algorithm is used to calculate the reflection and transmission coefficients, as well as the absorption rate of terahertz (THz) electromagnetic waves in time-varying plasma. The relation between the frequency of the THz wave and the propagation characteristic influenced by rise time, electron density, temperature, and depth of time-varying plasma plate is analyzed. Results demonstrate that the reflection coefficient is mainly influenced by plasma rise time and electron density. The absorption rate increases with decreasing rise time, increasing depth and electron density. Furthermore, the THz electromagnetic wave is an effective tool for the communication of reentry vehicle and high density plasma diagnosis because of its strong penetrability in high density plasma.