After the oily sludge is collected and heated by steam, a suitable number of additives like demulsifier and active agent are added to it, so as to break the emulsified state of oily water. After demulsification, a high-efficiency LWS series three-phase decanter is adopted to realize the continuous separation of oil, water, and slag in the oily sludge, and the water separated and the oil-water mixture not demulsified are directly sent to the sewage treatment system for treatment. Then, an LW series two-phase decanter with high efficiency is used for the process of dehydration. The crude oil residue in the dehydrated slag phase can be recovered by the combustion or thermal desorption process. After that, the separated crude oil can be collected and purified by a 3 phase disc stack separator to get purified oil which can be reused.