The invention provides a method for fixing CO2 and treating oil field sewage by using microalgae. The method comprises the following steps: a, firstly, performing oil removing pretreatment on oil field sewage in an oil field sewage union station by using an exhaust gas flotation method, feeding the oil field sewage of which most crude oil is removed through air flotation into an activated sludge method or biological membrane method oil removing treatment system for further removing crude oil, and adding N and P nutritive salts according to a weight part ratio of TOC:N:P being 100:(5-10):1 in the activated sludge method or biological membrane method oil removing process; b, introducing flue gas of which the content of CO2 is 3-15% into the oil field sewage after aerobic biological method treatment in the step a; then, feeding into a race track type microalgae culture system, adding microalgae seeds into the race track type microalgae culture system, growing for 5-15 days until the density of microalgae is greater than 10<7>/ml and the grease content is greater than 20%; c, feeding the alga-containing oil field sewage treated by the race track type microalgae culture system into a flocculation settling pond, collecting microalgae by using a flocculation method, filter-pressing the algal sapropel by using a filter-press to make alga cakes, extracting biodiesel oil from the alga cakes; filtering, sterilizing and deoxidizing settling effluent, and refilling the settling effluent back to a stratum to remove oil.