The formation conditions,the influence on weld properties and the control mechanism of delta ferrite in duplex stainless steel weld are discussed. The results show that the weld structure of duplex stainless steel is a duplex structure of austenite with different morphologies distributed at grain boundaries and in grains of ferrite matrix. Among the influence factors of delta ferrite,the chemical composition and cooling rate of weld metal play a decisive role. Excessive amount of delta ferrite in the weld has an adverse effect on the corrosion resistance and crack resistance of the welded joint. The optimum ratio of austenite to ferrite in weld used in engineering should be determined by procedure qualification method. Optimizing the chemical composition of the weld is the necessary condition to control the austenite formation in the weld,while the reasonable process methods and welding parameters are the sufficient conditions to control the austenite formation.