Photoinduced anisotropy(PIA) in amorphous $As_2S_3$ thin film, deposited by vacuum evaporation, is investigated. PIA is induced by linearly polarized $Ar^+$ ion laser beam(λ=514.5nm) and probed by weak $Ar^+$ ion laser (λ=514.5nm) and He-Ne laser (λ=632.8nm) beam through the crossed analyzer. Keeping pump beam intensity constantly, rotation of pump beam polarization direction induces reorientation phenomena of anisotropic axis. Introducing directional factor into simplified 3-level system, which is used to analyze photodarkening phenomena, analytical representation of PIA is derived. Temporal behavior of PIA and its reorientation phenomena are investigated and compared with theory. Pump beam intensity is 100mW/㎠ and thickness of a-$As_2S_3$ thin film in 3㎛. In that case time constant of photoinduced anisotropy obtained by method of least square curve fitting is $4.0×10^{-2}/sec$. The time constant of PIA we obtained is larger than that of photodarkening ($2.8×10^{-2}/sec$) reported.
