The invention relates to an observation window device arranged on a tunnel boring machine (TBM). The outline of the observation window is a disc; the observation window successively comprises a glass cover, lenses, a blank cap and a safety plug from front to back, and the periphery thereof comprises a base board flange, a positioning flange, a supported flange, a sheath and a strengthening ring. The base board flange is a hollow flange ring and is in combined welding with a cabin door by using the cabin door as a base; the glass cover, lenses, the blank cap, the safety plug, the positioning flange and the supported flange are coaxially installed and fixed by screws; and all joints are sealed by rubber gaskets and the like. The device applied on a shield machine is special equipment established for observing the state of the structure of the soil body in a soil chamber before builders need to enter the soil chamber to manually remove barriers and replace tools, and judges whether opening the door and entering the chamber is safe or not according to the observed soil body in the soil chamber so as to prevent disasters for the operating personnel entering the chamber and the whole construction site caused by collapse, onrush water, onrush sand and the like of an excavation face. The device has simple structure and easy use.