The invention pertains to an observing device of a polysilicon furnace. The observing device is arranged on the base of the viewing port of a furnace body and consists of a base, a bearing, an electric-control motor, a conveyor belt, a rotary shaft, a rotating seat, an infrared thermo detector probe, a camera and a viewport observing mirror, wherein, the base, one end of which is fixedly providedwith the electric-control motor, is fixedly connected on the base of the viewing port of the furnace body; the electric-control motor drives a motor wheel and a shaft wheel which is coaxial with the rotary shaft by the conveyor belt; the lower end of the shaft wheel is connected with the base by the bearing while the upper end of the shaft wheel is provided with the rotating saddle, on which the infrared thermo detector probe, the camera and the viewport observing mirror are arranged. Since the observing device is arranged on the base of the viewing port of the furnace body and the situation in the furnace body can observed on an operating worktable all the time, the safety of operators can be guaranteed and the observation is accurate and comprehensive with time and labor saved; in addition, the observing device has the advantages of simple structure, low manufacturing cost, convenient use and good effects.