We present a photothermoelastic model for studying the photothermal effect in chalcogenide glasses. The temperature rise and thermoelastic expansion for a model material As2S3 glass are calculated. Our result indicates that significant expansion (e.g. >1 µm) on the free surface of glass can be induced with only moderate temperature rise (e.g. <30 K). This occurs because of the Poisson effect of the large compressive forces generated on the interface between the irradiated and unirradiated areas in the material. We argue that photoexpansion in chalcogenide glasses arise from the concurrent effect of photothermoelastic expansion and photofluidity. The temperature dependence of photoexpansion is also discussed. The present model provides an explanation for the generality of photoexpansion in these materials.