The 3-D method is used to calculate the ground total electric field of two crossing HVDC transmission lines. By the comparison of the calculated results and the measured data on the crossing test lines with the bundles of 4×95 mm², the method is verified. Then the 3-D method is applied to the problem of practical crossing ±800 kV UHVDC transmission lines, including the distribution analysis of the voltage gradients on the conductor surface, the ground nominal electric field and the ground total electric field. Finally, the influence of the lower conductor height and the vertical polar distance on the maximum of the ground total electric field is discussed, and the minimal values of the lower conductor height and the vertical polar distance are recommended to meet the requirement of the electricmagnetic environmental protection standards. The results and approaches can provide technical support for the design of crossing HVDC and UHVDC transmission lines.