Abstract: Photoemissivity is an important factor to the measurement accuracy by optical thermometry for the surface of continuous casting billets, while the oxide skin thickness affects greatly the photoemissivity in the measuring process. According to the theory of electromagnetic wave propagation, a multilayer model was developed for the surface radiation of the billets to investigate how the oxide skin thickness affects the photoemissivity. A recursion formula was deduced for the photoemissivity of polarizing angle from billet surface, where the physic parameters of different layers were given as shown in relevant literature. The calculated results revealed that the photoemissivity is significantly increased when the horizontal polarizing angle is in the range from 60° to 80°. However, the photoemissivity comes to be stabilized after the oxide skin thickness increases to a certain extent. The two conclusions will benefit theoretically the instrument layout for surface thermometry in site.