The invention discloses a combined method of microalgae culture and waste gas denitration. The method comprises: (1) providing part of a carbon source for the microalgae culture process by using gas containing CO2 at least; keeping the algae liquid alkali when the culture is finished by means of microalgae metabolism; separating out microalgae from the harvested algae liquid to obtain microalgae and alkali residue; (2) transforming NOx in industrial smoke into nitric acid and/or nitrous acid; and (3) contacting the alkali residue obtained in the step (1) with the nitric acid and/or nitrous acid obtained in the step (2) to obtain a nitrogen-containing neutralized liquid, which provides a nitrogen source for the microalgae culture process. According to the method, 'microalgae culture' and 'waste gas denitration' can be combined reasonably, the microalgae can grow normally with NOx in the waste gas, the NOx can be effectively removed from the waste gas, and mutual influence of 'microalgae culture' and 'waste gas denitration' can be further avoided, so that the two processes can run stably and reliably.