The invention relates to a weather resistance detector for an external heat insulation system of an exterior wall. The weather resistance detector comprises an instrument control system, a recording and electrical control system, an environmental cabinet, a humiture measurement system, a dehumidifying and drying system, an air refrigeration system, an air heating system and a circulated water cooling and sprinkling system, wherein an observation device is further mounted in the environmental cabinet, and an observation frame is mounted on the observation device and can move up, down, left and right according to actual need. According to the weather resistance detector, durability detection can be carried out in the environmental cabinet through artificially simulating climatic conditions such as hot, cold and rainwater of natural environments, the observation device in the environmental cabinet can work in severe environments such as hot, cold and rainwater environments and can be used for carrying out real-time observation and subarea photographing on entire finishing coats, and then, a plurality of subarea images are synthesized into one image by adopting an MATLAB software image processing tool, so that deterioration characteristic symptoms such as cracking, bubbling and spalling of the finishing coats can be accurately and timely snapshot, and the working efficiency can be obviously increased.