Abstract: Aim To investigate the rectangular prism used as aberration - free laser attenuator under different incident conditions. Methods Ray tracing of geometric optics based on the reflection and refraction laws.Results Distances between incident and emergent beams on the prism surfaces and tolerant apertures were derived under differ- ent conditions,which can be used as design reference of such a king of attenuator.Conclusion According to the apertures of the attenuated beam and the prism. aberrationfree attenuation can be realized and interference effect can be avoided with adequate selection of incident conditions.