GaP IR thin films were deposited through RF magnetron sputtering with a GaP disk as the target. The intensity of Ar I 750nm optical emission line was kept the same in different experiments, with which various deposition parameters were got, and the deposition processes were studied with computer simulation. The results showed that under low sputtering power and high working gas pressure, the sputtering yields and transporting efficiencies of Ga and P atoms as well as their energies at arriving the substrate's surface are small, the sputtering yield and transporting efficiency of Ga atom are both larger than those of P atom, hence the depositing rate of GaP film is low, the content of Ga in the film is larger than that of P, which makes the film have high absorbance in IR waveband. But under high sputtering power and low working gas pressure, the atoms' sputtering yields, transporting efficiencies and energies at arriving the substrate's surface all increase, the sputtering yield of Ga atom is larger than that of P atom, whereas its transporting efficiency is smaller than that of P atom, hence the depositing rate of GaP film is high and stoichiometric film with low absorbance is deposited, which is advantageous to depositing large-thickness GaP film.