Abstract: DC arc fault in a photovoltaic (PV) system is difficult to detect due to the negative resistance, which threatens the safe operation of the system. We propose an approach to detect the DC arc fault in the PV system using the spectrum integration difference of the parallel capacitor current. An off-grid PV experiment platform is built, and capacitors are paralleled with each branch in the PV system. The capacitor currents are monitored by Hall current sensors. The amplitude, pulse polarity, and spectrum characteristics of the capacitor currents when series or parallel arc faults occur at different locations in the system are analyzed, and they are compared with the capacitor current characteristics of the grounding fault and switch operation. The spectra of the capacitor currents within 0.5 ms pre-fault and post-fault are integrated, and the spectrum integration differences are calculated. The research results indicate that, the capacitor current amplitudes of the grounding fault and switch operation are much higher than those of the arc faults. The spectrum integration differences of the arc faults are within the range of 1.1×10⁴~2.5×10⁴ A·Hz, which can be used as the detection criterion for the DC arc fault. The polarity of the first pulse of the capacitor current indicates the relative location of the fault in the branch where the capacitor is paralleled with, thus the arc fault can be localized.