To assess the shielding property of carbonyl iron material at terahertz wavelengths, the transmission spectra of reference samples, 0.5 mm carbonyl iron samples and 1 mm carbonyl iron samples were measured respectively by a Terahertz Time-domain Spectroscopy (THz-TDS). The analysis showed that with the increase of the thickness of the carbonyl iron samples, the pulse width of the terahertz time-domain spectrum would increase, its peak value would decrease and the frequency domain spectral range would be reduced. The carbonyl iron samples had their maximum shielding effectiveness of about 66 db. The wavelength ranges in which the 0.5 mm and 1 mm carbonyl iron samples had the shielding effectiveness greater than 66 db are those from 0.76 to 1.46 THz and from 0.49 to 1.48 THz respectively.