In the field of precision electrical measurement, there are few special coaxial scanner for AC automatic test systems. Aiming at this, a 4-wire-15 channel all-coaxial scanner device is designed. The device is based on the precision motion control of the stepper motor, and drags the fast-plug-in coaxial axis component to do X-Y bidirectional positioning motion, realizing the reliable automatic switching of the coaxial measurement channel. By independently designing the flexible docking mechanism, the risk of channel docking error and irreversible physical damage caused by movement misalignment are eliminated. By analyzing the generation mechanism of parasitic parameters such as channel thermal electromotive force, leakage impedance, parasitic capacitance and inductive coupling potential, the key design parameters affecting the electrical characteristics of scanner are defined, and then the targeted optimization design method is proposed. The test results demonstrate that the thermal electromotive force in the scanner channel is less than ±50 nV, the leakage impedance of the channel to the ground reaches 1014 Ω, and the parasitic capacitance between the channels is less than 1×10-5 pF, additionally, the inductive coupling coefficient between channels is less than 1.2×10-10 H.