The movement of free metal particles within the cavity of a direct current gas insulation transmission line (GIL) can significantly reduce the gas insulation level, and in extreme cases, it may cause partial discharge or surface flashover. The motion states of metal particles under a uniform DC electric field and their identification methods are investigated. An experimental platform is established for the movement of metal particles under DC voltage and ultrasonic detection. Through extensive experiments, the uplift voltage of metal particles between parallel plates, their motion trajectories, and the ultrasonic signal waveforms generated by their movement are recorded. The experimental results indicate that under positive polarity voltage, linear metal particles exhibit a unique state of lower plate flying firefly movement, with a higher tendency for this movement in linear metal particles with larger diameters and greater lengths. A partial discharge experimental platform of alternating current pulse is also set up to measure the partial discharge patterns of metal particles in different motion states. The results confirm that corona discharge is one of the main factors contributing to the lower plate flying firefly movement. Noise reduction processing is applied to the ultrasonic signal waveforms of metal particle movement collected by the experimental setup, and three characteristic parameters of the ultrasonic signal are extracted for comparison across different particle motion states. The findings suggest that the parameter threshold method can effectively differentiate between the size of spherical metal particles and the operational states of linear metal particles.