Photoinduced changes in refractive index and film thickness of amorphous Sn1As20S79 semiconductor film are studied experimentally. The emperical rules in as-evaporated，annealed and well-illuminated states are obtained respectively. An ultraviolet irradiation technique is presented and employed successfully to fabricate a Sn1As20S79 stripe waveguide, which shaws good characteristics of a waveguide under the 632.8nm guided mode excitation.