Abstract: The main factors which have influence on the welding performance and welded joint mechanical performance of steel bar electroslag pressure welding are the flux composition and high-temperature physics performance of the slag. A kind of fused flux has been developed based on the study of these factors.This flux is suitable to steel bar electroslag pressure welding for the class Ⅲ steel bar as the middle Mn and Si type fused flux,and has been extensively applied to building engineering.