The corona effect of HVDC transmission lines affects the electromagnetic environment significantly. A numerical method to calculate the ion flow field caused by corona effect in the vicinity of multi-circuit HVDC transmission lines is described. Deutsch assumption, a commonly used simplification in traditional numerical methods is eliminated. Skills of initial value estimation and boundary process are introduced to improve the stability and efficiency of calculation. Bundle conductors and ground wires are taken into account in this simulation model. The ion flow field of a plusmn500 kV HVDC project with bipolar lines on the same tower is simulated. The electrical field and ion current density on ground level are compared among different line arrangements.