This work addresses damage evaluation of porous mullite refractory subjected to thermal shock. Incommunicating circular pores were distributed randomly at a volume percentage up to 40% in a cylinder of 20 cm diameter. The analysis was performed by means of the software ANSYS® combined with a pre-program that generates randomly distributed pores of given size. The analysis procedure was divided into two stages. In the first, transient thermal analysis considering temperature-dependent material property was dealt with different thermal shock temperatures under natural cooling condition. The following structure analysis ran based on the obtained temperature distribution. The material damage was defined by that the local tensile stress reached to or was over the strength of the refractory. The extent of damage was determined as the ratio of the area of the damaged regions to the section area of the cylinder. The results show that the porosity, thermal shock temperature and cooling time have a high effect on the material damage. The lower the porosity is, the larger the extent of damage. The thermal damage increases with the raise of thermal shock temperature and the cooling time. The damage develops rapidly within 10 minutes but slows down after one hour cooling. The damage difference at high shock temperature stage (≥ 1000°C) is less than at low shock temperature stage. The pore size effect gets into practice only at high shock temperature stage: the damage increases with the raise of the pore size. The present research confirms that high porosity and small pore size could decrease greatly thermal shock damage and should be considered in the micro structural design of refractory.