| Electric vehicles (EVs) have attracted more and more attentions because of their advantages of energy saving, environmental protection, low carbon emission and so on. In order to improve the transmission efficiency of EV wireless charging system and optimize the coil design, an equivalent current-source circuit model with two-coil series-series topology is analyzed, and four optimal aspects, i.e. optimal coil turns, optimal coil side length, shorter transmission distance and optimal load, are achieved to improve the transmission efficiency of the planar spiral square coils using the mutual inductance, the coil equivalent series resistance and the load. It is verified using the COMSOL software. The optimal simulation result shows that: 1) there is an approximately optimal relationship between the coil side length d and the transmission distance D, i.e. d=2D; 2) there is a set of optimal parameters for the EV wireless charging system, i.e. D=45cm, d=90cm, coil turns N=30, load RL= 80Ω and thus system transmission efficiency achieving 88.36% so as the high efficiency transmission of EV wireless charging is realized.