Cyclohexanone oxime electrosynthesis from low-concentration NO is important but suffers from low Faraday efficiency and current density because of severe competitive hydrogen and ammonia evolution reactions. Here, porous carbon-supported ultrasmall Ag nanoparticles are designed to achieve 85.5% FE at a partial current density of 100 mA cm⁻² for cyclohexanone oxime with a 3% NO concentration, outperforming pure Ag nanoparticles. Mechanistic studies reveal that porous carbon can optimize the NO transport path and H₂O molecule distribution on the catalyst surface, accelerating the reaction dynamics for cyclohexanone oxime electrosynthesis and suppressing ammonia and hydrogen formation. This work paves the way for accessing low-concentration gaseous reactants for various catalytic reactions.