Application of cationic polymer as flocculating agent

The invention discloses an application of a polymer as a flocculating agent. The structural formula of the polymer is shown in the formula I in the specification, wherein in the formula I, m is a natural number from 2888 to 4129 when being the number-average degree of polymerization; n is a natural number from 7684 to 27280 when being the number-average degree of polymerization; m is a natural number from 4393 to 6397 when being the weight-average degree of polymerization; n is a natural number from 11686 to 44601 when being the weight-average degree of polymerization; the flocculating agent is used for removing oil in oily sewage. In the application, the cationic polymer has the characteristics of good water solubility, convenience in use and the like, and is especially suitable for solving the problem of low removal rates of emulsified oil and suspended solids in the oily sewage produced in heavy oil fields. Besides, the flocculating agent also has the advantages of small dross production and low content of moisture and high content of oil in dross.