The invention relates to a capacitive electric power equipment dielectric loss on-line detection method and a capacitive electric power equipment dielectric loss on-line detection system. On-line measurement of current of a grounding wire of capacitive electric power equipment and voltage of capacitive electric power equipment bus voltage transformer is realized. The coefficients of the subharmonic components of the acquired voltage and the acquired current are acquired by Fourier analysis. The amplitudes of the subharmonic components of the acquired voltage and the acquired current are calculated according to the coefficients of the subharmonic components of the acquired voltage and the acquired current. According to the amplitudes of the subharmonic components of the voltage and the current, the equivalent capacitance and the equivalent resistance of the capacitive electric power equipment are calculated. A tangent of a dielectric loss angle is calculated according to the equivalent capacitance and the equivalent resistance of the capacitive electric power equipment, and is output. Equipment and means of original voltage measurement and original current measurement are not required to be changed, and the dielectric loss of the capacitive equipment is calculated accurately without requiring accurate synchronization, and then a difficulty of voltage and current synchronous measurement is prevented, and in addition, the accuracy of the measurement of the dielectric loss is guaranteed even under a condition of unsynchronized measurement of voltage signals and current signals.