Liquid-solid fluidized bed electrodes (FBEs) are an important technical route for the preparation of composite powders because of their large activation area and fast mass transfer rate. In this work, the reaction mechanism of 3D electrode prepared from composite powder in liquid-solid fluidized bed was studied in the Cu/Ni system, and two reaction paths were proposed, namely, particle electrodes (main reaction) and cathode feeder electrode (side reaction). Mathematical model with voltage drop as the main parameter was determined. Key conductivity test was conducted to support the measurement of voltage drop data. The sum of the voltage drop of bed particle reaction and bed particle intrinsic was successfully less than that of the voltage drop of bed solution phase and cathode feeder electrode reaction. Therefore, FBEs can be used to prepare Ni-coated copper composite powders with different particle sizes. When the bed height is 2 cm with 100 mu m copper powder as the particle electrodes, the voltage of the FBE reaction path is reduced by approximately 0.55 V. These results showed that the FBEs have achieved the reaction path for preparing composite powder and thus are of great importance for large-scale preparation.