A novel ultrasonic-vibration-composite electrical-discharge-machining （EDM） small-hole high-speed machining system is designed for small-hole machining of difficult-to-machine metals， such as nickel-based superalloys. The system can simultaneously realize the functions of electrode ultrasonic vibration， electrode rotation， and water flush out at the center of the electrode. It can be easily employed for other EDM machines. A series of small-hole machining experiments are carried out on the superalloy GH4099. By altering the machining current and time， indexes such as the material removal rate （MRR）， tool electrode wear rate （TWR）， and hole morphology are compared. The experimental results reveal that ultrasonic-vibration-composite machining can effectively improve the MRR and reduce the relative TWR during low-current machining. Under the same conditions， when the peak current is 1 A， the hole depth processed by this system is 89.62% higher than that of ordinary EDM. When the peak current is 6 A， the MRR increases by 46.42% compared with that for ordinary EDM. The electrode wear rate is reduced by 25.85%. Thus， we can conclude that the proposed system features a notably stable machining process that provides a convenient and efficient solution for EDM finishing of difficult-to-machine metals.