The present invention provides a process of cultivating microalgae and a joint method of same jointed with denitration. During the microalgae cultivation, EM bacteria is added into the microalgae suspension. In the nutrient stream for cultivating microalgae, at least one of the nitrogen source, phosphorus source and carbon source is provided in the form of a nutrient salt. During the cultivation, the pH of the microalgae suspension is adjusted with nitric acid and/or nitrous acid. The joint method includes (1) a step of cultivating microalgae; (2) a separation step of separating a microalgae suspension obtained from step (1) into a wet microalgae (microalgae biomass) and a residual cultivation solution; and (3) a NOx absorbing/immobilizing step of denitrating an industrial waste gas with the residual cultivation solution obtained from step (2). The nutrient stream absorbed with NOx obtained from step (3) is used to provide nitrogen source to the microalgae cultivation of step (1).