The magnetic hydrodynamics(MHD) model which is used to describe the heat and fluid flow inside the plasma torch under atmosphere condition was presented. Numerical simulation of the plasma torch with convergence-divergence nozzle shape was carried out by using a two-dimensional mathematical model. Plasma temperature, velocity, pressure, Mach number contours,which describe the characteristic of heat and fluid flow inside the plasma torch, were obtained. Results show that the plasma field can be divided into three regions named subsonic region, transonic region, and the supersonic region.