For the Ximeng-Beijing east section of 1 000 kV Ximeng-Nanjing ultra high voltage transmission line, this paper calculated the electric unbalance degree and best transposition position of the mixed transmission line with single-circuit and double-circuit on same tower based on ATP-EMTP. The results shows that the best transposition point is not the 1/3 or 2/3 equal position of lines. Basing on electric unbalance degree, this paper recommends that the lines has a full transposition in single-circuit sections without transposition in double-circuit sections with negative phase sequence, and two transposition towers are located at 63.14 and 103.13 km.