The unbalanced current caused by the asymmetry of un-transposed transmission lines increases the loss of the transmission components of a power grid. Moreover， the negative sequence current may lead to cascading tripping of a number of generators as well. Based on the individual phase modeling of 220 kV un-transposed transmission lines， a simplified model of un-transposed transmission lines is proposed. It can be used for unbalanced current research， and its applicability is proven by theoretical calculation and simulation studies using EMTDC. Based on the asymmetry of mutual inductance matrix of transmission lines， the analytical relation between mutual inductance matrix and sequence currents can be derived. Furthermore， the possibilities of reducing the negative sequence current by adjusting the elements of mutual inductance matrix are studied as well. The results from calculation and simulation show that the proposed method can effectively restrain the negative sequence voltage and current caused by unbalanced parameters of transmission lines.