In this paper,200nm molybdenum oxide (MoO₃) terahertz modulation film deposited on silicon wafers were prepared by thermal evaporation.The terahertz modulation characteristics of MoO_3films under different excitation optical powers were s tudied by Fourier spectroscopy and terahertz time-domain spectroscopy (THz-TDS) at room temperature.The important optical parameters extraction model of thin film is p roposed.The time-domain signal of thin film is measured by transmission THz time-domain sp ectroscopy. The transmittance and modulation efficiency of THz wave in film and change of co mplex dielectric constant of MoO_3film are calculated respectively.The results show that the terahertz modulation depth of MoO_3film increases with the increase of laser p ower with the wave length of 980nm.The minimum transmittance achieved 61% and the maximum modulation factor achieved 10% under 266mW laser power at 0.23THz.Analyzing th e complex dielectric constant and carrier density of MoO_3film shows that the ex citation of carriers led to the change of dielectric constant,enhance the conductivity of t he film,and reduce the transmittance of THz wave in the film.The experimental data are prov ided for the application of MoO_3film in terahertz band modulation.