Double arcing phenomenon is a limit to increasing the capacity of the plasma cutting torch. In an attempt to enhance the ability of being invulnerable to the double arcing, a double nozzle structure is introduced in this paper. The reason why the double nozzle structure is less vulnerable to the double arcing phenomenon than single nozzle structure is explored. Double nozzle structure allows the longer nozzle which may cause stronger shock wave. In order to evaluate the influence of shock wave on the cutting ability, the influence of nozzle length on the double nozzle structure plasma arc is investigated. The modeling results show that the longer nozzle produces the stronger shock wave outside the nozzle outlet, but the energy flux and momentum flux become concentrated after the shock wave and increases with the increasing of nozzle length. So the double nozzle structure improves the cutting ability of the plasma torch and meanwhile be less vulnerable to the double arcing phenomenon.