The breaking capacity of vacuum circuit breakers limits its application in the field of high-voltage and high-current breaking. Obtaining plasma parameters in the arcing process is very important for improving the breaking capacity of VCB. The emission spectroscopy is used to diagnose the plasma parameters in the vacuum arc in this paper. The axial distribution of electron temperature, electron density, and spectral line intensity in the vacuum arc under different current amplitude conditions is studied. The relationship between the diffusion process of different particles in the vacuum arc and the diameter of the arc column is analyzed based on the high-speed pictures of the vacuum arc. The electron temperature is 8000-10 000 K, and the electron density is 10¹⁹-10²⁰ m^-3. The electron temperature and electron density gradually decrease from the cathode to the anode. The intensity of the copper atomic line is mainly concentrated at the two poles, while the intensity of the monovalent copper ion line gradually increases from the cathode to the anode. The radial distribution of the copper atomic line intensity presents a flat-top wave distribution, and the monovalent copper ion line intensity presents a Gaussion-like distribution. The spectral range of copper atoms is slightly larger than the diameter of the arc column, and the spectral range of monovalent copper ions is slightly smaller than the diameter of the arc column. The diffusion speed of the two particles is different.