A same tower multi-parallel line model of phase-splitting parameter is built with EMTDC， which aims to the serious unbalanced current on a Shenzhen 220 kV modernized four-parallel line on the same tower replacing a double-parallel transmission line. Based on study on influenced factors of unbalanced current such as: phase sequence arrangement， line power flow and operation in loop network， suggestions for limiting unbalance current are proposed. Simulation result shows that the strong electromagnetic coupling among each circuits is the primary reason to cause the unbalanced current， and the unbalance of concerned line in the loop network is an important factor as well. Increasing space among lines， reducing the difference of line power flows， adopting negative phase sequence array and loop network open operation under certain conditions are effective measures to limit the unbalance current on a multi-parallel line on the same tower.