Abstract: Al₂ O₃ coating was prepared on carbon steel surface by atmospheric plasma spraying（APS）. The influence of different coating thickness on the solar absorptance to infrared radiation（IR） emittance ratio of Al₂ O₃ coating was analyzed. In addition, scanning electron microscopy（SEM） was used to characterize the pores in the coating, and the influence of porosity and pore morphology on the solar absorptance and IR emittance ratio of Al₂ O₃ under the same coating thickness was studied in detail. The results show that, with the increase of the thickness of coating, the solar absorptance to IR emittance ratio of Al₂ O₃ coating decreases and finally tends to be stable. The solar absorptance to IR emittance ratio of Al₂ O₃ coating increases first and then decreases with the increase of porosity. When the porosity of the coating is in the range of 6%-10%, the pores in the direction parallel to the matrix have a great influence on the solar absorptance to IR emittance ratio. When the porosity of the coating is more than 10%, the pore size of the coating is the main factor for IR emittance ratio.