Abstract: In the electrical discharge machining (EDM) of small holes with a high aspect ratio, as the depth increases, debris removal becomes difficult, leading to decreased machining stability and limiting further improvement in the aspect ratio. To address this, a negative pressure assisted electrical discharge machining (NP-EDM) method is proposed in this study. By promoting the formation of inter-electrode bubbles under a negative-pressure environment and combining it with internal flushing through a tube electrode, debris removal is enhanced. The experiment showed that, under the same machining parameters, when machining for 700 s, the aspect ratio of small holes produced by NP-EDM increased by 30.01% compared to that under atmospheric pressure conditions. High-speed photography revealed that under negative pressure, the number of cavitation bubbles in the gap increases, and they tend to coalesce and escape rapidly. Characterization of the hole walls revealed that NP-EDM produces smoother hole walls and suppresses sidewall discharge. This method improves the inter-electrode flow field and debris removal capability, providing a new approach to breaking through the aspect ratio limit in deep-small-hole EDM.