The ions generated by corona discharge of high voltage direct current（HVDC） transmission lines can strengthen the electric field intensity in the space around the lines.In order to accurately calculate the ion flow field

the finite element-integral method is applied to solve the control equations of bipolar ionized field.The finite element extrapolation is employed in the calculation of the resultant electric field

and the initial value of space charge density is improved.Application of the proposed method to calculations of the coaxial cylindrical model and the ±400 kV DC transmission lines verifies its effectiveness.Moreover

an actual ±500 kV DC transmission lines is calculated with this method

and the result is compared with those from the existing algorithms

as a result

higher accuracy of the proposed method is observed.In addition

the proposed method is adopted to predict the resultant electric field intensity and ion current density at the ground level under ±800 kV DC transmission lines.