Three-order perturbation theory is used for the study of monocrystalline GaAs. Under an assumption of a simplified but more realistic band structure of GaAs crystal, formulas of three-photon absorption coefficient are obtained. And further more, after the escape process of an excited electron being considered, formulas of thr ee-photon photoemission coefficient on a NEA GaAs photocathode are derived. The comparisons of theoretical calculated values with experimental ones are given. The experimental values are obtained by using a 2.06 μm ns pulse Q-switched laser. The theoretical values fit the experimental ones satisfactorily.