An ultrasonic probe was introduced into the kettle of subcritical water extraction device to extract volatile oil from Lithospermum erythrorhizon. The effect of temperature, pressure, ultrasonic power, and frequency on the extraction yield was studied. Gas chromatography-mass spectrometry (GC-MS) was applied to the analysis of the compositions of volatile oil. The mechanism of ultrasonic enhanced subcritical water extraction (USWE) was discussed. The results showed that the ultrasound-assisted enhancement effect of 20 KHz was better than that of 36 KHz and increased with output power (0250W). The subcritical water extraction yield increased from 1.87% to 2.39% via ultrasonic oscillation (250W, 20 KHz) at a temperature of 160 degrees C and a pressure of 5MPa in the 25-minute extraction. Nineteen components were identified chiefly consisting of 18 carbon unsaturated fatty acid methyl esters, hexadecanoic acid methyl ester and pentadecane. Mechanism of USWE was cavitation and mechanical effect.