The invention discloses a method for improving the viscosity of a polymer solution prepared from oilfield sewage, belonging to the field of oilfield sewage treatment. The oilfield sewage is filtered by a filter or directly enters an ultrafiltration system to remove macromolecular organisms, bacteria and the like, the oilfield sewage is treated by ion exchange equipment to remove high-valent positive ions seriously influencing the viscosity of the polymer solution, and the dissolved oxygen in the sewage is removed through a deoxygenation device. The method is mature in technology and simple to operate and easy to implement; the treated sewage is used for preparing a polymer solution, thus the initial viscosity of the polymer solution is greatly improved; moreover, the viscosity stability of the polymer solution is guaranteed, and the treatment cost is saved.