The invention discloses an apparatus for culturing microalgae by using waste water, the apparatus comprises a tank, the tank is divided into a microalgae culture chamber and a waste water fermentation chamber, the waste water fermentation chamber is positioned at lower part of the microalgae culture chamber, a gas transmission channel is arranged between two chambers, a liquid inlet and a drain outlet are arranged on the sidewall of the waste water fermentation chamber, a slag removal opening is arranged at bottom, a filtering apparatus communicated to the drain outlet is arranged at external part; a microalgae culture rack is arranged in the microalgae culture chamber, an air exhaust opening is arranged at top, a spray thrower is arranged over the microalgae culture rack, and the spray thrower is connected to the filtering apparatus through an extracting pump. The tank is divided into two parts, waste water fermentation is carried out at lower part of the tank, carbon dioxide gas and a broth can be obtained, and the broth is injected to the microalgae culture chamber through the filtering apparatus, the extracting pump and the spray thrower for microalgae culture. integration of waste water fermentation and microalgae culture can be realized, microalgae culture cost is reduced, so that nutrient element in waste water can be fully used by microalgae.