The radiation characteristics of high-temperature gas flow have important application in detection, discrimination and aerothermal environment prediction for hypersonic vehicles. Based on a band model and considering the visible and infrared radiation of the ablation products of silicon-based materials for heat-shield, the method and code were developed to compute spectral parameter of ablation products and the radiation characteristics of high temperature gas flow field. The flow field parameters resulted from the numerical simulation and used the code, the radiation characteristics of flow over a model in a free-flight ballistic range and hypersonic blunt body were computed, and particularly the ablation effects were studied. The research shows that the ablation products have important effects on infrared radiation of the flow field.