The foaming process of flame retardant polymer was characterized by thermogravimetric analysis, differential scanning calorimetry and Fourier transform infrared spectroscopy, and the products were analyzed qualitatively and quantitatively at the micro level. Besides, one side heating tests under open fire were conducted on double layer concrete panels, illustrating the fire resistant effect of polymer composite with multiple geometries in cement based materials. The results show that the flame retardant polymer experiences three intensive weight loss stages, coupled with the formation of a honeycomb like carbonization structure. Under the open fire, different flame retardant polymers can extend the “temperature platform” of cement based composite panels through the process of melting, polymerization and foaming, and reduce the heat transfer rate to the internal structure in the way of heat conduction to a certain extent. The foaming products can overflow and cover the fire surface of the structure, giving full play to the fire resistance of tunnel lining structure.