The utility model discloses a capacitive equipment dielectric loss online monitoring system which comprises a data acquisition unit and an online monitoring unit, wherein the data acquisition unit comprises a zero flux sensor and a voltage signal acquisition card, the zero flux sensor is used for acquiring leakage current, and a small current signal is converted into a small voltage signal which accesses to the locally mounted voltages signal acquisition card; and the voltage signal acquisition card converts a voltage analog into a digital and uploads the digital into the online monitoring unit. According to the capacitive equipment dielectric loss online monitoring system, with the adoption of data acquisition optical fiber transmission and an IEC61850-9-2 transport protocol, real-time performance and stability of data are guaranteed, and transmission of mass data is ensured; and besides, the signal acquisition unit uses a high-precision synchronous acquisition card for amplifying secondary output voltage of the sensor and converting the voltage into the digital, so that the output voltage signals of the sensor system are effectively increased, the signal-noise ratio is increased, influence of external interference signals on the measuring result is reduced, and the testing system has higher stability.