The purpose of this research was to establish an effective method for extracting phenolic compounds from Eucommia ulmoides leaves. Seven different deep eutectic solvents (DESs) were prepared, and ultrasonic-assisted technology was employed to optimize the extraction parameters. Factors such as the DES molar ratio, water content, liquid-to-solid ratio, ultrasonic duration, temperature, and power were systematically investigated. The optimal extraction conditions were determined to include a choline-chloride-to-ethylene-glycol molar ratio of 1:4, 30% water content, a liquid-to-solid ratio of 40:1 mL/g, an ultrasonication time of 48 min, a temperature of 53 °C, and ultrasonication power of 60%. Under these optimized conditions, the yields of phenolic compounds and flavonoids reached 17.16 mg/g and 48.23 mg/g, respectively, which were significantly higher (p < 0.05) than those obtained by traditional extraction methods. These findings indicate that the use of ultrasonic-assisted DES extraction notably improved the content of active compounds and the antioxidant properties of the extracts. Fourier transform infrared spectroscopy and scanning electron microscopy analyses revealed that this method promotes the release of active compounds by disrupting the integrity of the cell walls. This research offers a theoretical foundation and practical guidance for the efficient utilization and advanced processing of E. ulmoides leaves.