In this paper, the expansion process of metal wire electrical explosion in the air was observed by using shadow photography technology. The influence of applied voltage and metal wire type on the expansion velocity of the discharge channel was analyzed. The experimental results show that the channel expansion speed is the fastest in the vaporization and ionization stages of the metal wire. The shock wave experienced a process of gradual transformation from a cylindrical wave to a spherical wave. The wave velocity decreased rapidly at first, then decreased slowly, and finally tended to be gentle. As the applied voltage increases, the energy storage of the system increases, the phase transition in the process of wire electrical explosion is advanced, and the channel expansion speed and shock wave velocity increase. The energy conversion efficiency of shock wave energy of fusible metals such as aluminum, copper, and nickel-chromium are higher than that of refractory metal tungsten due to the less energy required for phase transformation, and the channel expansion velocity and shock wave velocity are higher than those of refractory metals.