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