The study presents a method for online measurement of metal droplets during vacuum arc discharge, based on Mie scattering theory. A vacuum arc metal droplet online measurement system is constructed using a 650-nm red laser as the light source and a camera-synchronous control device. The particle size distribution is inverted using a nonindependent mode differential evolution algorithm. The accuracy of the measurement system was validated through experiments with four types of polystyrene latex ball standard particles, showcasing a measurement error within 5%. Further, online measurements of metal droplets were conducted using four different cathode metal materials under varying discharge currents and pulse widths. The findings suggest that for a given metal, alterations in discharge pulse width or discharge current do not remarkably affect the size of the metal droplets. Conversely, under identical discharge current and pulse width conditions, the material of the cathode metal markedly influences the particle size distribution. Specifically, under the same operational conditions, the median sizes of aluminum, iron, titanium, and molybdenum droplets are 1.55, 1.25, 1.15, and 0.75 μm, respectively. These experimental results serve as a valuable reference for further study of the vacuum arc cathode discharge process.