GaAs is an important material for the fabrication of semiconductor heterostructure lasers working at terahertz frequencies. To study the optical properties of p-type GaAs in far-infrared region, p-type (Be-doped) GaAs with carrier concentrations varied from 1.54×1015～ 1.85×1019 cm-3 has been epitaxially grown by gas source molecular beam epitaxy (GSMBE) on semi-insulating GaAs(100) substrates. Fourier transform infrared (FTIR) spectrometer is employed to characterize far-infrared reflectance spectra of the p-type GaAs. Simulation of the measured far-infrared reflectance spectra has been performed. And refractive indices, extinction coefficients and absorption coefficients for different carrier concentrations have been determined. It is found that extinction coefficients and absorption coefficients increase with the carrier concentration in far-infrared region, and the maximum of absorption coefficients can reach 4.0×104 cm-1.