Terahertz spectra modulation can be potentially used in the remote sensing. The spectra modulation of terahertz radiation from two plasmas is demonstrated experimentally. With the comparison of the spectra of terahertz radiated from single and two plasmas, the output spectrum of terahertz wave has been proved to be of interference superposition of the two separate terahertz waves. With measurement and analysis of the polarization states of the output terahertz wave, it is fund that the two orthogonal components of THz electric fields have effects on the components involved in the interference of two terahertz waves. The output terahertz radiation from two plasmas is simulated, and the result shows that the distance between two plasmas contributes greatly to the spectrum modulation of terahertz radiation.