The formation conditions of acicular ferrite, the influence of acicular ferrite on weld toughness and the control mechanism of acicular ferrite of X80 pipeline steel weld are discussed. The results show that the weld microstructure of X80 steel is composed of a large number of acicular ferrite plus a little proeutectoid ferrite. In the factors affecting acicular ferrite,the chemical composition and cooling rate of the weld play a decisive role. There is a corresponding relationship between the morphology and quantity of acicular ferrite in welds and weld toughness,the effect of welding heat input on weld toughness is more complex and there is an optimal value for heat input. The optimized weld alloy system and chemical composition are necessary conditions to control the acicular ferrite formation in the weld,and the reasonable procedure methods and welding parameters（including heat input） are sufficient conditions to control acicular ferrite formation.