Transmission lines in some areas may have no condition to transposition completely, because of a shortage of line corridor. And un-transposed multi-circuit transmission lines will cause electrical three-phase parameters asymmetry. To solve this problem, this paper takes a 750 kV double circuit lines without transposition for an example, and the formulas of calculating the voltage imbalance on double-circuit lines are presented. Using PSCAD/EMTDC, the zero-sequence voltage unbalance and negative sequence voltage unbalance are calculated in different types of towers, different phase sequence arrangement, different distance between the two loops and different line length. According to the simulation results, when the transmission line is short, the voltage imbalance can be controlled within a certain range by adopting with the reverse phase sequence in the same tower window, which loops distance is adjusted properly. To avoid bad effect on the power system, transmission line must be transposed when the line is long.