To study the impact of coupling circuits in high speed motorized spindle’s dynamic mathematical model on load characteristics of the spindle, according to invariance principle of control theory, external decoupling branches is introduced into the point where the difference between reference current and feedback current of the high speed motorized spindle is, to eliminate the impact of coupling voltage, and then the impact way of excitation current changes and torque current changes on the spindle’s flux, electromagnetic torque, and load characteristics are studied when the load varies. Meanwhile, the load characteristics are tested on the test platform of high speed motorized spindle. Theoretical analysis and experimental results show that the decoupling effect of high speed motorized spindle directly affects the stability of the spindle’s flux and the linear tracking ability of the torque current on load, which is reflected by the parallelism of mechanical characteristic curve under different frequencies, the pulse level of electromagnetic torque, the load capacity of the spindle, power factor, efficiency and other parameters. Therefore the spindle’s actual decoupling results and dynamic performance under the control mode can be judged and forecasted, according to the trends of mechanical characteristic curve, power factor curve and efficiency curve.