The invention discloses a mechanical seal leakage rate online monitoring system, and belongs to the field of a mechanical seal monitoring system. The mechanical seal leakage rate online monitoring system comprises a leakage-sensitive pipe, a fluid infusion tank, a circulation tank, a data processor, a first liquid level transmitter, a second liquid level transmitter, an automatic control switch valve, a fluid infusion pump, a non-return valve, a mechanical seal structure and a water tank. In case of normal work, the automatic control switch valve is normally-closed, the circulation tank communicates with the leakage-sensitive pipe, and liquid level change of the leakage-sensitive pipe sensitively reflects the leakage rate of the mechanical seal structure; when the liquid level of the leakage-sensitive pipe is reduced to a certain degree, the automatic control switch valve is open, the fluid infusion tank, the circulation tank and the leakage-sensitive pipe form a communicating vessel so as to realize automatic fluid infusion of the leakage-sensitive pipe; and when the liquid level of the fluid infusion tank is reduced to a certain degree, the fluid infusion pump is started, and thus automatic fluid infusion of the fluid infusion tank is realized. The monitoring system provided by the invention can sensitively monitor the leakage rate of a mechanical seal in real time and output early warning signals when the leakage rate exceeds a standard; and when a seal blocking liquid is insufficient, the seal blocking liquid can also be automatically supplemented.