In this paper, the reasons for the formation of "white stripe" in the heat affected zone beside low alloy steel of FB2-30Cr1Mo1V dissimilar steel welded joint are analyzed: the microstructure of "white stripe" is tempered sorbite and the grain is fine. The main cause of "white stripe" is composition segregation which hinder grain growth during welding thermal cycle, resulting fine and corrosion-resistant microstructure generated. The peak temperature of welding thermal cycle is main influence factor. Thermal simulation technology was used to reproduce the "white stripe", which appeared at the peak temperature between 900°C and 1050°C.