In order to select wires and transformers， their technical and economical operation range are analyzed using the economical current density and load rate. Based on the definition of the conjugate slip current sensitivity to the line parameters， the sensitivity of network loss on line parameters is derived. According to the size of the network loss sensitivity， the degree of influence of network parameter on power loss can be determined， which provides an easy way of estimating the network loss after the parameters are changed. With minimum comprehensive cost of the investment and power loss as the objective function， a method is presented to optimize the network parameters with the particle swarm algorithm. According to the economical operation range of lines and transformers， the optimization interval is selected. A particle location library is established and the location of particle is secondarily updated. Simulation results show the simplicity， feasibility and practicality of the proposed algorithm.