Abstract: The submerged arc welding (SAW) sy stem consists of an inverted welding power supply with constant current output character, a wire feeder, a welding traveller and a controller.Taking the arc voltage and the setting voltage as input parameters, the controller gives a DC output, which is modulated in pulse width modulation scheme, to drive and adjust the wire feeding speed.A mathematical model related to the welding arc and the wire feeding speed during SAW is established in the form of a first order equation.The model described the balance relation between burning the procedure of the wire in the arc and the feeding of the wire via the wire-feeder.According to thermodynamic analysis of the power supply, molten pool and wire, a scheme for the selection of the wire diameter is obtained, on basis of the diameter of the wire being in proportion to the square root of the welding current.A detailed equation between the wire diameter and thewelding current is reported.