A large amount of sludge produced in the processing of crude oil has brought great harm. In this paper, common flocculants are summarized. According to the treatment process of crude oil, flocculants are divided into organic flocculants, inorganic flocculants, microbial flocculants, composite flocculants, and magnetic flocculants. The mechanism and limitation of each flocculation on crude oil sludge were analyzed. Finally, it is pointed out that the flocculation process of sludge is complex, and the development of new flocculant with high efficiency, safety, and economy is the main trend of flocculant development in the future.