One dimensional multigroup radiation transport program (RDMG) is use d to simulate the process of Xray heating Au disk. It is found that although the outward spectrum from Au disk is near Planckian,the peak region is in some way deviated from Planck spectrum. This is due to the fact that the corona plasm a is optical thin to the frequencies of peak region ,which is corresponding to the valley of the absorption coefficient spectrum between the peak of N band and that of O band．