The invention aims at providing a passive containment cooling system steam discharging device. The device comprises a cooling water box, a water seal tank and an exhaust pipe, wherein the exhaust pipe comprises an inlet section bend, an exhaust straight pipe and an outlet section bend which are connected with one another in order, exhaust pipe holes are formed in the side walls of the cooling water box and the water seal tank, the exhaust straight pipe is arranged between the exhaust pipe hole of the cooling water box and the exhaust pipe hole of the water seal tank, the inlet section bend with an upward opening extends into the exhaust pipe hole of the cooling water box, the outlet section bend with a downward opening extends into the exhaust pipe hole of the water seal tank, an outlet hole is formed in the end part of the outlet section bend, water is stored in the cooling water box and the water seal tank, the upper end surface of the inlet section bend is above the liquid level in the cooling water box, and the uppermost outlet hole of the outlet section bend is below the liquid level of the water seal tank when steam does not enter the exhaust pipe. The device provided by the invention can be automatically and reliably switched on without depending on external power, and the flowing resistance is small to guarantee that steam produced in a PCS (Passive containment cooling system) can be smoothly, stably and reliably discharged.