Due to the random distribution of micro-structure on the surface, the actual metal has obvious masking effect in light reflection or thermal radiation. The existing bidirectional reflection distribution function model using piecewise geometric attenuation factor is difficult to obtain high characterization accuracy. In this paper, first, the masking and shadowing effect of the inclination difference of adjacent micro-facet on the metal surface of the incident and outgoing light are analyzed. Then, the existing integral masking function is modified based on the geometric relationship between the inclination angle of micro-facet and light, the distribution of micro-facet on the metal surface is characterized by the Cauchy distribution, and a polarization model of spontaneous emission on the metal surface based on the masking function modification is proposed. Finally, the polarization degree of spontaneous emission of different metal materials under heating conditions is obtained through experiments to verify the effect of the modified model. The results show that the spontaneous emission model based on the masking function modification is more consistent with the experimental data than the existing model.