A numerical model is constructed based on the solution of the magnetohydrodynamic equations within the framework of phase field algorithm to simulate the metal transfer process and investigate the mechanism of the transition of metal transfer from globular to spray mode. Surface tension is the strongest driving force acting on the pendent droplet in globular transfer, while the governing force shifts to the electromagnetic pinch force in spray transfer. Driving force balance in the axial direction could be the indicator of detachment in globular transfer, while that force balance doesn't exist in spray transfer. The condition for the transition from globular to spray transfer is that the local pressure at the root of the droplet caused by the electromagnetic pinch force exceeds the surface tension pressure at the droplet tip corresponding to a droplet radius equals to the wire radius. Compared with volume of fluid method, phase field method shows a more physically realistic estimation of the current path from the drop to the arc plasma and leads to a better agreement with experimental data.