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Highlights It is obtained that ultrasound could not only reduce the viscosity of heavy oil, but also increase its viscosity under different irradiation time. The addition of tetralin could stabilize the reaction system and effectively reduce the recovery of the system after ultrasound. We used the Transform Infrared Spectrometer (FTIR) and Gas Chromatograph (GC) to analyze the chemical compositions and structural changes of oil samples before and after ultrasonic irradiation, and gave the possible mechanism of viscosity changes.