An on-line monitor for direct currents on a neutral point down conductor of a transformer belongs to the technical field of electrotechnics and is used for monitoring the direct currents on the neutral point down conductor of the transformer in an on-line manner. The on-line monitor is composed of two circuits. Integrated circuits A1 to A5 and related elements form a measuring emission circuit, and integrated circuits A6 to A10 and related elements form a receiving display circuit. Two points P1 and P2 are taken from the neutral point down conductor of the transformer, and resistance Ro between the two points is made to be 0.5 milli-ohm. A direct current Ix passing across the resistance Ro is a to-be-measured value. Two diodes D1 and D2, connected reversely, are connected across the two points P1 and P2. An output frequency of a pin 3 of the integrated circuit A5 is a pulse voltage of Fx, and the Fx is positively proportional to the Ix. A red laser pen D5 is driven by the pulse voltage to emit a light pulse. A light signal is converted by a photosensitive diode D6 into an electric signal. Through processing of the circuits A6 to A10, etc, a direct current Vo is output from an output end of the A10, and the Vo is positively proportional to the Ix.