The optimal design of loosely coupled transformer is the key technique in the wireless power transfer (WPT) system. An improved reluctance model is proposed based on partitioning flux tube method, all parts of reluctance and coupling coefficient expressions are shown. This paper shows the optimization design method which objective function is minimum copper loss in given power transfer capability. In the light of features of optimization model, primal-dual interior point method is introduced to opti- mize the design parameters. Experiment results show that the mutual inductance calculated was verified with experimental measurement. The WPT system has less copper loss and less number of turns in given power transfer capability. The system achieves low cost low loss design, all the parameters met the design requirements. The validity of optimization algorithm is proved, and can be used as a general WPT optimization design method to other types of loosely coupling transformers.