The invention discloses an online diagnosis device for a tap joint of a distribution transformer. The diagnosis device comprises a voltage stabilizing circuit, the voltage stabilizing circuit is connected with a signal converter, the output end of the signal converter is connected with an A/D converter, the A/D converter is connected with an isolation circuit, the isolation circuit is connected with a single-chip microcomputer, the output end of the single-chip microcomputer is connected with a display device and the isolation circuit, the isolation circuit is connected with a power carrier wave communication circuit, and the power carrier wave communication circuit is connected with an external AC power supply. On the premise that the power supply system is stable, the most reasonable tap joint is provided; the voltage stabilizing circuit is added before the signal converter, voltage which fluctuates originally is made smooth, it is not required to consider whether there is a power filter capacitor in the surrounding of the diagnosis device, and attenuation of 127kHz high-frequency signals of the power carrier wave module caused by filtering is avoided.