Deep-small holes with internal featuring microstructures are widely used in aerospace and gas turbine industries; but their special internal geometries pose significant challenges to traditional machining methods. To address the fabrication difficulties of internal surface microstructures in deep-small holes, a local gas cavity-assisted multi-jet electrochemical machining method was proposed in this work. Through visual observation and process experiments, the modulation behavior of gas-liquid multiphase flow fields in deep-small holes were investigated, along with the formation mechanisms of hole internal surface microstructures via multi-jet processing were explored. Results indicated that using an optimized tubular cathode design and flow parameters enables the formation of stable non-submerged parallel multi-jets within deep-small holes. The parallel electrolyte jets allowed localized efficient material removal from the inner walls of deep holes and follow the same fundamental principles as conventional electrochemical jet machining. Various deep holes with complex inner-surface microstructures were successfully fabricated. This method may provide new technical solutions for manufacturing internal microstructures in gas turbine turbulated cooling holes.