In a contactless power transfer (CPT) system, it is impossible to accomplish primary and secondary part design independently by the traditional mutual inductance coefficient, which results in design difficulty of system power and efficiency. Therefore, a new method is proposed to calculate power and efficiency. Based on a new definition of mutual inductance coefficient, power and efficiency of the secondary part which is compensated in both series and parallel circuit are inferred, and the design of magnetic structure is decoupled. By analyzing the relation of power and magnetic parameter, such as winding number, quality factor and sectional area of secondary coil, it is found that the transmission power and the efficiency of the secondary part are independent of the number of the secondary winding. Moreover, the primary and secondary parameter design flow is given, and the optimization principle and method for transmission power and efficiency are presented, which are beneficial to the size reduction of magnetic core and the increasement of transmission power and efficiency. Finally, the prototype is developed and the validity of the optimization method is verified by the simulation and the experiment.