We report the first experimental demonstration of a novel plasma diagnostics method using the single-shot THz detection method. In this experiment, a broadband single THz pulse is generated by focusing two-color (400 nm and 800 nm in wavelength) laser pulses in air and the THz pulse is sent through an inductively-coupled plasma with a density of 1013–1014 cm−3 which is in the modern tokamak plasma density range. Phase shift of the THz pulse is measured by the single-shot THz detection method and the instant plasma density information is obtained from the phase shift. The result is compared with that of the existing THz time domain spectroscopy method and it is found that both results are in good agreement. It implies that the proposed method in this paper can provide a new plasma diagnostics tool which can be used for dynamically changing plasmas.