To upgrade residual oil, the residual oil was subjected to ultrasonic treatment with an output of 800 W, at a temperature of 70 °C and at intervals ranging from 0 to 11 minutes. The experiment illustrated that 7 minutes of ultrasonic treatment reduced viscosity and carbon residue by 14.1% and 7.4%, respectively. This also produced an increase in saturates content and a decrease in the content of aromatics, resins and asphaltenes. Furthermore, the parameters of the average molecular structure were characterized by FT-IR, and 1H-NMR, while an increasingly greater change in the parameters were produced by an ultrasonic treatment interval of up to 7 minutes. The mechanical stirring and cavitation from ultrasonic treatment caused a series of changes in the molecules of residual oil. Microscopic changes affected the parameters of the average molecular structure, as usually shown in SARA fractions. The changes in the thermal reaction properties of residual oil after ultrasonic treatment were analyzed by thermogravimetry. As the ultrasonic treatment time increased, the apparent activation energy needed for pyrolysis decreased gradually, as did the temperature.