In order to optimize the design of limited-size wireless power transmission system coil, improve the transmission efficiency of the system, the equivalent circuit model and transmission coil model parameters of the dual coils string-string topology were studied by the method of modeling simulation and experimental verification of the transmission coil by ANSYS software, the main influencing factors of the optimization of the plane spiral coil under the condition of dimensional constraints were obtained, then took the quality factor as the standard, used ANSYS software to research and design the transmission coil part, found the best winding way under the size constraints. Finally, the coils were winded according to the optimization results of theory and simulation, and the feasibility of theoretical research and design was verified by experiments. The results show that, compared with the two control coil groups, the transmission efficiency of the optimized coil group was increased from 0.85 and 0.87 to 0.97, which was increased by 14.1% and 11.4% respectively. It can be seen that under the condition of dimensional constraint, the coil winded according to the optimization results of the theory and simulation can effectively improve the transmission performance of the system.