Numerical analysis has been conducted to characterize thermal plasma generated by a novel long direct current (DC) arc discharge system for waste treatment. Effects of arc current, gas flow rate, confinement tube diameter, and electrode gap distance on the temperature distribution of thermal plasma were investigated. From the numerical results, the design variables were expected to control over waste treatment processes effectively in the long DC arc thermal plasma. A broad high temperature distribution with low power consumption was predicted in the case of large diameter of the plasma confinement tube, because the less intrusion of cold gas into the arc column leads to the expanding of current density profile in the radial direction. In the downstream region, the fraction of low temperature area to cross sectional area that hinders the decomposition of wastes was decreased with increasing the electrode gap distance.