It is of great significance for indoor radon radiation protection to study the mechanism of radon migration systematically caused by heat-moisture coupled transfer in building walls. The radon migration model of porous building walls under the influence of the heat-moisture coupled model is built. On the base of it, four types of different temperature and humidity conditions, including high temperature and high humidity, high temperature and low humidity, low temperature and high humidity and low temperature and low humidity, and the real ambient temperature, relative humidity conditions in a southern city of China in July, are set for the numerical simulation. The migration mechanism of the radon through the wall influenced by outdoor temperature and humidity conditions, including its emanation, diffusion and condensation, is studied by numerical simulation, and the main factors leading the results are analyzed, which provides some reference for indoor radon radiation protection.