A cold dielectric high-temperature superconducting (HTS) cable has multilayer transmission conductors to carry a large current. When the conductor layer number increases, the unevenly distributed currents caused by the skin effect in each conductor layer become more and more nonuniform, and would lead to the increased amount of loss and poorer transmission performance. In order to fully exploit the performance of conductors in cables, it is important to understand the current sharing mechanism between layers. An optimal design model for minimizing current difference in each conductor layer was proposed using an improved particle swarm optimization algorithm. A 1 km, 110 kV/3 kA cold dielectric HTS cable consisted of YBCO tapes and with current evenly sharing was optimally designed based on an equivalent circuit model. After optimization, we find that the maximum unbalanced rate of the current in conductor layers is 4.1% and there is a uniform current distribution among the conductors in shield layers. The current calculation result of the designed HTS cable shows the current is evenly shared in different layers and thus this optimal design method is validated.