<p>In traditional machining processes, surface microcracks are prone to generate due to the hardness and brittleness of quartz glass. A hybrid process of electrochemical discharge machining (ECDM) and high-speed milling (HSM) is proposed for efficiently machining quartz glass without surface microcracks. During ECDM-HSM process, a “mushroom cloud” gas film is found by a high-speed camera, which facilitates to constraint discharges to a tool–electrode end. Machining experiments of ECDM-HSM obtains the processing effect of key factors of tool rotation speed, applied voltage, and feed rate. The higher rotation speed requires higher critical voltage to start discharges. The high feed rate can improve machining efficiency and quality significantly owing to the combined action of plastic cutting, discharges, and chemical reaction. The homogenization mechanism of discharging energy and the transition mechanism of plastic cutting are proposed for illuminating the processing effect. The matching mapping of key parameters is established for optimizing the processing parameters. As an example, a regular groove without surface microcracks is machined with the processing efficiency improved up to 20 times as compared with that of HSM.</p>