The invention relates to a muffle furnace, a sealed cap and a tool for operating sealed cap thereof. The sealed cap of the muffle furnace comprises a sealed cap body, and a temperature-measuring opening which is communicated with the outside world formed on the sealed cap body. The temperature-measuring opening is capable of opening a first dam-board and a second dam-board in a sliding mode respectively, wherein the first dam-board is covered with transparency, the second dam-board is used for blocking the volatile matter of heating elements, and the second dam-board is attached to the first dam-board. The sealed cap of the muffle furnace has a first work position and a second work position, in the first work position, both the first dam-board and the second dam-board cover the temperature-measuring opening, in the second work position, the second dam-board removes the temperature-measuring opening, then the temperature measurer is capable of measuring the temperature inside the muffle furnace through the first dam-board. Through the above technical scheme, the sealed cap is not only capable of detecting the temperature inside the muffle furnace and the service condition of the heating elements in real time, but also avoiding the influence from the heating elements in the muffle furnace and the volatile substance produced by silicon carbide rods on the observation of the situations inside the furnace, and then largely improving work efficiency.