The vacuum switch used in intermediate frequency (360~800 Hz) power system of more/all electric aircraft is being developed, and the theoretical understanding of the arcing process at intermediate frequency is increasing. The continuum emission theory of arc plasma was analyzed, electron temperature and electron density of intermediate frequency (400~800 Hz) vacuum arc were obtained by application of a simplified optical-channel model and detection on the intensities of dual-narrowband continuum emission spectra. Then, a plasma-parameter diagnostic system was designed to implement the above measurement. This system consists of a color CCD camera with a dual-narrowband filter and arc-motion picture analysis software, and it can record the arc appearance and the data for measurement simultaneously. After calibrated by using standard source of radiance temperature, the system was applied to observe the arc mode evolution and measure the plasma parameters. The results confirmed the validity of analysis and measurement in this work. At intermediate frequency, the transition arc mode and the diffuse arc mode exist during arc evolution. The transition arc evolves rapidly, and converts into the diffuse arc at the peak current. At the current peak of 8 kA with different frequencies, the electron temperature varies from 0.5~3 eV, and the electron density ranges from 1020~1021 m-3, which are in agreement with those achieved by other researchers.