In the theoretical and experimental research of radiation transport in inertial confinement fusion (ICF), the radiation spectrum in the foam cylinder can be changed by the surrounding high-Z wall. In this paper, we simulated and studied the property of flux emitted by the high-Z wall using 1-D plane model. Our work shows that the gradient of electron temperature is the reason why spectrum of flux emitted by the high-Z wall deviates from equillibrium. In order to quantitatively study the property of flux emitted by the high-Z wall, we define the non-equilibrium rate and deduced its estimating formula in the temperature scale of ICF experiment.