Abstract: Transition metals can efficiently adsorb reactants and catalyze various chemical reactions. With advantages such as low cost and abundant reserves, they represent the most important and promising material system for replacing precious metal catalysts. However, significant challenges remain in optimizing preparation processes and enhancing catalytic performance. This work employed the carbon thermal shock (CTs) process to synthesize FeCoNi@CNTs medium-entropy alloy nanoparticles (MEA NPs) catalysts on carbon nanotube films (CNTs) substrates within an ultra-short time scale. This process features short processing time and low energy consumption. By adjusting CTs process parameters, the size of MEA NPs was reduced and their composition homogenized. Results indicate that when the CTs duration is 60 ms (60 ms-FeCoNi@CNTs), the synthesized MEA NPs exhibit FCC structure with the smallest size and most uniform element distribution. Both excessively short or long CTs durations lead to larger particle sizes, reduced numbers, and uneven element distribution. Oxygen evolution reaction (OER) testing revealed that the 60 ms-FeCoNi@CNTs exhibited outstanding OER performance and long-term operational stability in 1 mol/L KOH electrolyte, with an overpotential of only 300 mV at a current density of 10 mA/cm². Smaller MEA NPs correspond to higher specific surface areas, exposing more active Fe, Co, and Ni sites. Conversely, extending or shortening CTs increases MEA NPs size and reduces their number,correspondingly decreasing exposed active sites and degrading OER performance. This work confirms that CTs represent an efficient and rapid method for preparing MEA NPs, offering a novel strategy for designing high-performance OER catalysts.