Metal droplet is produced accompanied with vacuum arc discharge, which is important to the research of cathode spot and the application of vacuum arc. The droplet comes from the cathode spot crater and can reflect the physical process of the cathode spot. However, it will destroy the uniformity of surface deposition in engineering and should be avoided as much as possible. The measurement of metal droplet usually adopts off-line collector, which cannot obtain the signal of the whole space and singe arc. In order to on-line measure the droplet, a new method by the Mie scattering is developed in this work, and its feasibility is investigated. The characteristic of the scattering light of titanium droplet is computed by the simulation code. The results indicate that the scattering light beams of the small droplet are distributed at all angles. With the increase of the diameter, the scattered light beams are more and more concentrated in the forward direction, which allows the inversion of the signals of the droplets with different diameters. Then the detector is designed with different annuluses. When the detector is divided into 35 annuluses, the light energy coefficient matrix is easy to solve and the measurement system has a good resolution. The experimental setup is built and the preliminary experiment is carried out. The results indicate that the diameters of titanium droplets are mainly around 9.8 μm, which verifies the effectiveness of the Mie scattering method of measuring vacuum arc droplets. However, the small droplet information is not detected, so the droplet diameter distribution is quite different from the off-line measurement. The reason is that the signal-to-noise ratio of the measurement system is poor, thereby leading the scattered signals of the small droplet to fail to be obtained effectively. The experimental setup need to be further optimized.