Given the problems of oil refinery slag sludge containing a large amount of dirty oil, complex composition, and challenging to deal with, the pretreatment test of slag sludge produced by oil refinery of Sinopec Jinling Branch was used by ultrasonic means.The influence of ultrasonic power, ultrasonic temperature, ultrasonic time, and ultrasonic frequency on the decontamination rate of slag sludge was analyzed by L₁₆ (4⁴) orthogonal test method. Error analysis was carried out by SPSS 25.0, and the orthogonal experimental results were refined by using the single-factor test. The characteristics of sludge change before and after ultrasound were characterized by infrared spectroscopy, scanning electron mirror, energy spectrum analysis, and so on.The results are as follows:The orthogonal test showed that the influencing order of factors was ultrasonic power < ultrasonic time < ultrasonic temperature < ultrasonic frequency. When the ultrasonic power was 60 W, ultrasonic time was 10 min, the ultrasonic temperature was 60℃, and ultrasonic frequency was 3 times, the removal rate of petroleum substances could reach 31.13%. The infrared spectrum showed that the corresponding peak of scum oil sludge after ultrasonic treatment was significantly changed, showing that aromatics, alkanes, olefins, and other substances were decreased, indicating that the long chain of petroleum hydrocarbon was destroyed. After ultrasonic extraction, the alkanes in the extracted oil decreased and the alcohols increased, which may by that one of the hydrogen groups of alkanes was replaced by hydroxyl group to produce corresponding alcohol. Compared with alkanes, alcohols were more easily degraded by microorganisms. Scanning electron microscope (SEM) analysis showed that after ultrasonic treatment, the gap between the particles increased, the viscosity decreased, and the texture became loose.The mechanism of petroleum hydrocarbon in scum oil sludge by the ultrasonic method was as follows:through mechanical effect, cavitation effect and thermal effect, the long chain of petroleum hydrocarbonin scum oil sludge was destroyed, the viscosity between particles was reduced, the voids between particles were increased, the three-phase separation of oil, mud and water was promoted, and the petroleum hydrocarbon was decomposed and separated, thus reducing the content of petroleum substances in scum oil sludge. Energy dispersive spectrum analysis(EDS) showed that the proportion of carbon and oxygen elements in the oil sludge after ultrasound did not significantly decrease, indicating that the ultrasonic thermal effect did not occur obvious gaseous oxidation reaction. As a result, the ultrasonic method is a physical method, with no additional chemical drug, and the treatment method is simple. It can be used as a composite treatment process pretreatment means.