Abstract: To enhance the dynamic and performance testing of subway pantograph-catenary systems, particularly in the standard measurement of arcing, a fiber optic spectrometer-based in-situ spectral testing system has been developed. This system is designed for real-time measurement of the characteristic spectra of spontaneous and intermittent arcing in subway operations. This paper presents the design and laboratory wavelength calibration of the subway pantograph-catenary arcing spectral in-situ testing system. Subsequently, real-time experiments were conducted at subway operation sites. By using in-situ measurement data from a specific Beijing subway line and integrating train operation monitoring parameters, the characteristic spectra of pantograph-catenary arcing were extracted and analyzed. The results indicate distinct spectral features of subway arcing, differing from natural sunlight and ordinary lighting, in the range of 220–225 nm with a peak wavelength at 224.6 nm. These findings provide empirical data to address discrepancies in spectral characteristic wavelengths between international and national standards and academic papers, offering significant guidance for the selection, development, and calibration of China’s subway pantograph-catenary arcing monitoring systems.