Abstract: Based on thermodynamics of welding metallurgy,the overlaying welding electrodes of SMAW were studied.The content of alloying element Me in welding electrode was controlled by adding ferroalloy,pure or its carbide contained in covering of electrodes.For the highor middle carbon W-Cr-Mo-V and Cr-Mo-V-Si type electrodes building up with basic covering,the transition factors of W,Mo,Cr ,V,Siwere in the range of 0.8~0.9 As an alloying element,carbon picked up in the weld metal was under the influence of graghite shape,and its trainsition factor ranged between 0.3~0.6 ,because of its stronger deosidization at the welding temperature,As demonstrated in this paper,the general formula on the weight ratio of covering of electrodes must be modified according to the content of transition metal in covering.