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Electrochemical micromachining (EMM) has become a popular technology for fabricating microscale structures. To obtain higher machining localization and minor stray corrosion, a new hybrid EMM process combining air assistance and ultrasonic assistance was introduced in this paper. Simulations and experimental investigations were conducted to study the influencing mechanism and the influencing characteristics of the assisting process energies. In the multi-physics model, a reactant transport model is included, and the direct coupling between the assisting processes and the electrochemical process is achieved. Results indicate that, when the two assisting processes are applied separately, the assisting effect tend to act on the entire workpiece surface, either promotes the overall dissolution or inhibits it, usually leading to unsatisfactory results; by combining the two assisting processes, synergistic effect occurs: dissolution in the central area is promoted by the ultrasonic assistance, while stray corrosion in the margin area is hindered by the air assistance. Thus, microstructures with enhanced aspect ratio and reduced stray corrosion can be fabricated. The results also show that the surrounding airflow can produce an electrical insulating effect on both the workpiece and the cathode. When the air pressure is low, an additional concentrating effect on the electric field can be observed; however, a high air pressure leads to a greatly hindered dissolution. When the applied amplitude is appropriate, ultrasonic assistance promotes the electrochemical dissolution through disturbance effect and cavitation phenomena. But if the amplitude is too large, massive cavitation bubbles will inhibit the dissolution greatly. [ABSTRACT FROM AUTHOR]