Abstract: Total electric field strength and ion current density are considered as two of principal environment indices of HVDC transmission lines,and should meet the electromagnetic environment requirement during operating.When the power lines pass the fog zone,the ion flow field will become more severe.In this paper,the mechanism of the effects of fog on ion flow field was analyzed,and an approach calculated the ion flow field with fog was presented.In the case of Xiangjiaba-Shanghai ±800kV UHVDC power lines,the effect regularity of fog on ion flow field was researched,and the analysis results show that the total electric field strength near ground increased observably with the increasing of fog density,and the higher density of fog,the greater the total electric field strength,while the ion current density keep invariant basically.Compare with the value under fair weather condition,the total electric field strength increase about 45% under serious fog condition.