The invention relates to an on-line monitoring device for leakage of a nitrogen-charging fire-extinguishing oil discharge valve of an oil immersed transformer. The on-line monitoring device comprises the oil discharge valve arranged on an oil discharge pipeline. The oil discharge pipeline is provided with a viewport located below the oil discharge valve. An oil receiving groove is formed in the inner wall of the oil discharge pipeline. An opening of the oil receiving groove is upward and is arranged in a spiral shape with a high end and a low end. One side wall of the oil receiving groove is attached to the inner wall of the oil discharge pipeline. The oil receiving groove is located below the oil discharge valve. An oil collection box is arranged on the portion, below an oil outlet, of the low end of the oil receiving groove and located in the viewport. A liquid level detection unit is arranged in the oil collection box and connected with a signal sending module. The signal sending module sends liquid level information to a remote monitoring terminal. Due to the fact that the liquid level detection unit is arranged in the oil collection box and the liquid level information of leaked oil in the oil collection box is sent to the remote monitoring terminal through the signal sending module, a maintainer absent can be reminded remotely, and real-time monitoring on leakage of the oil discharge valve is realized.