The invention provides a method for processing oilfield wastewater and fixing CO2 (carbon dioxide) by using microalgae; the method comprises the following steps of: using oilfield wastewater in which the crude oil content is lower than 10mg/L, the TOC (total organic carbon) is lower than 40mg/L and the COD (chemical oxygen demand) is lower than 100mg/L for microalgae cultivation and carbon fixing, introducing CO2 directly into the wastewater, selecting the microalgae with high stress resistance, high oil content and high function in degrading petroleum hydrocarbon according to the water quality of the oilfield wastewater introduced with the CO2, combining the large-scale cultivation of the microalgae with the wastewater treatment, building a large-scale cultivation system by using five stepped microalgae processing ponds which are connected with each other, and then adjusting the water temperature to be 20 DEG C to 35 DEG C; a silicon carbide micro-filtration membrane is used for harvesting, and 10-20% of the harvested microalgae is used as algae seed for the algae liquid cultivation of the next batch and 70-90% of the harvested microalgae is used for preparing biodiesel. The wastewater temperature which is the most proper for the growth of the algae is controlled by controlling the content of fresh wastewater, and the full-year stable and continuous running of the oilfield wastewater processing and CO2 fixing system is achieved.