Terahertz time-domain spectroscopy (THz-TDS), which directly measures the THz wave’s temporal electric field, can give the amplitude and phase of the THz wave pulse simultaneously. THz-TDS is attracting more attention among scientists. InP with short carrier average collision time and low effective mass is growing up as one of the best photoconductive materials for emitting and detecting THz waves. An n-type InP of 0.35 Ω·cm was characterized over the range from 0.2 to 4 THz at room temperature in the present paper with THz time-domain spectroscopy, which was placed in a closed box purged with dry nitrogen gas. Some THz optical properties, such as complex refractive index, dielectric constant, and conductivity, were extracted, based on more exact iterative method with new initial function. Drude model was also applied for simulation, which fitted well with the experimental results. Finally, the carrier average collision time, density and mobility of the InP were also characterized.