Oil sludge from an oil storage tank was treated using a combined ultrasound and thermochemical cleaning method. The effects of cleaning temperature, cleaning time, ultrasound frequency, ultrasound power, and other factors on the oil recovery were investigated. Experiments on the screening and remixing of reagents were conducted and indicated that the best constituents of detergent solution were sodium silicate, sodium dodecyl benzene sulfonate, and fatty alcohol ethoxylates in a ratio of 1:1:1. The optimum conditions of the combined treatment system were also determined. The results showed that, under the optimum conditions, the oil content of oil sludge dropped from 43.13% to 1.01%, 0.53% of solids remained in the separated oil layer, and 99.32% oil recovery could be achieved when the concentration of detergent solution was to 2 g/L. Compared with traditional thermochemical cleaning, the oil recovery was 17.65% higher for the combined treatment system.