Surface emissivity of casting billets is an important parameter that affects the temperature measurement on the billets. In this paper, an experimental apparatus is established by the method of radiation energy comparison. The apparatus consists of a heating system, an angle rotating system, a temperature detection system, a vacuum control system and a background radiation shielding system. The emissivity of GCr15 steel at different angles, temperatures and oxidations is measured by this apparatus. Research shows that emissivity increases first and then decreases with the increase of the angle. Emissivity increases with the increasing oxidation, and the angel of the maximum emissivity is gradually reduced. In addition, emissivity increases with the increasing temperature but the influence of the temperature is limited once the temperature is over 1000 ℃. At last, the measurement uncertainty is analyzed and the maximum emissivity uncertainty of the deep oxidized GCr 15 steel is 0.0205 when the temperature is above 1000 ℃.