Drained sludge of Daqing oilfield No.4 oil production plant was collected from bottoms of tanks, and the oil content was usually lower than 2%. In this paper, the contents of petroleum substances, total petroleum hydrocarbons (TPH) and animal and plant oil were investigated. Ultrasound-UV method was used to detect the oil content of oily sludge. Results showed that the best ultrasound time was 8 min and the best solid-liquid ratio for the extraction was about 1:20. The standard curve was constructed. The comparison between the ultrasound-UV method and the traditional method shows that the ultrasound-UV method is more suitable for the detection of oil content of oily sludge with lesser oil. Moreover, the methods of detecting sludge with lower oil content have been established.