External thermal insulation systems often have durability problems, including cracking, hollowing, and falling off, which seriously affect safety and energy-saving effects. Based on finite element theory and using ANSYS software, this paper studies the distribution law of the temperature field and temperature stress of the external thermal insulation system. It was found that, compared with an uninsulated wall, the temperature stress of the substrate in summer was reduced by 52.9%, and the temperature stress of the substrate in winter was reduced by 50.9%. The temperature stress is mainly concentrated in the middle position of the external wall insulation system, and the middle of the wall can appear as a hollow drum and fall off. When the temperature of the external wall surface is 60 °C, the maximum temperature stress of the insulation system is 2.46 MPa, compared with the external wall surface of 70 °C—a decrease of 22.2%; the maximum temperature stress on the substrate is 0.46 MPa—a decrease of 20.7%. When the temperature of the outer wall surface is 50 °C, the maximum temperature stress suffered by the insulation system is 1.75 MPa, compared with the outer wall surface of 70 °C—a decrease of 44.4%. Meanwhile, the maximum temperature stress suffered by the substrate is 0.34 MPa—a decrease of 41.4%. This paper investigates and numerically simulates the durability of external wall insulation systems for buildings in hot summer and cold winter regions, and studies the durability of EPS insulation, which can provide guidance for other insulation material design and durability studies.