NOVELTY - Producing biodiesel by cultivating microalgae by using urban sewage comprises: (1) degradation of organic wastewater and culturing microalgae: photosynthesis of algae, using carbon dioxide (CO2) and sewage nutrients, including nitrogen and phosphorus synthetic algae for releasing substances or oxygen (O2); (2) collecting algae: algae are collected using filters to achieve algae water separation; and (3) generating biodiesel: microalgae are collected using microwave assisted extraction ultrasonication oil, methanol or ethanol, under catalytic basic conditions.    USE - The method or system is useful for producing biodiesel by cultivating microalgae by using urban sewage (claimed).    ADVANTAGE - According to the method and the system, a sewage treatment system is converted from a treatment technique to a production technique; high contents of organics and nutrients such as nitrogen and phosphorus in the urban sewage are conducive to mass propagation of the microalgae, so that the sewage is converted from pollutants to raw materials for the growth of the microalgae; CO2 circulation and O2 circulation are tightly combined through the establishment of an algal-bacterial symbiotic system, so that the growth of the microalgae is no longer dependent on an external carbon source, and the production cost is low due to the reduction of aeration systems in an aeration tank; the illumination in the aeration tank is abundant due to the use of a photosynthesis component, which is conducive to the photosynthesis of the microalgae; and the subsequent sludge quantity is small due to the use of efficiently-degraded strains.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is a system for producing biodiesel by cultivating microalgae by using urban sewage, comprising urban sewage settling tank, units for degradation of organic wastewater strains, symbiotic bacteria and algae cultivation of microalgae cells, collecting components, generating bio-diesel generating unit, and unit for symbiotic bacteria and algae strains with high efficiency degradation of urban sewage.