The utility model relates to a furnace cavity flame observation device of a coal burning boiler, in particular to a portable high-temperature inner observation mirror system, which is characterized by comprising a video camera, a protecting cover provided with a front extending pipe, a camera, a cooling device and a display system. The video camera is installed in a protecting cover inner cavity, and the camera is electrically connected with the video camera and installed in the front extending pipe of the protecting cover. The cooling device is connected and communicated with a connector of the protecting cover. A data interface of the video camera is connected with the display system. The front extending pipe of the protecting cover is capable of extending into a furnace cavity fire observation hole, and the pipe diameter is movably matched with the inner diameter of the fire observation hole. The portable high-temperature inner observation mirror system has the advantages of being capable of extending into any fire observation hole to achieve an observation point of combustion situations in the furnace cavity as a portable device, high in carrying and observing flexibility and large and random in observation range, thereby being high in observation efficiency.