Three-coil magnetic resonance wireless power transmission (MR-WPT) system with cross-coupling is in a detuned state on the primary side, resulting in a decrease in the transmission power of the system. However, the existing methods for studying the relationship between detuning parameters and transmitted power of MR-WPT systems have problems such as a large number of detuning parameter variables, complex relationships between parameters and large control algorithm calculations. To address the above problems, a three-coil series-connected MR-WPT system primary-side detuning parameter design method is proposed with a three-coil series-connected magnetic resonance detuning topology. This method simplifies the parameter calculation process by introducing a virtual coupling factor without complicated algorithms and designing additional hardware circuits. The influence of the primary-side detuning parameters on three-coil MR-WPT system is studied, and the relationships between different virtual coupling factors, detuning factors and system transmitted power in the three coil loops of the MR-WPT system are analyzed. ① The internal resistance ratio between transmitting coil and relay coil loops can illustrate whether there is frequency splitting in the system. ② When the virtual coupling factor value of transmitting coil and relay coil equals with the value of relay coil and receiving coil, the corresponding output power of the system is greater than the output power with unequaled values. The output power of the system is maximum when the virtual coupling factor value of transmitting coil and receiving coil is 1. ③ The detuning factor can measure the detuning degree of the primary side. The larger value means the greater detuning degree of the primary side and the smaller value means the smaller detuning degree of the primary side. ④ When the detuning factor value is fixed, the larger the quality factor is, the smaller is the difference between working frequency and resonance frequency as well as the detuning degree. When the detuning factor is fixed, the smaller the quality factor is, the larger is the difference between working frequency and resonance frequency as well as the detuning degree. When designing the detuning parameters of the primary side, it is suggested to choose larger quality factors to reduce the fluctuation of detuning frequency. The experimental results verify the correctness of the parameter design and analysis results.