The invention belongs to the field of microalgae cultivation, and particularly relates to a method for cultivating microalgae by using CO2 as a carbon source. Tris is added into a microalgae cultivating solution containing the CO2 gas serving as the carbon source, so that the concentration of the total inorganic carbon source in the cultivation solution is obviously increased, and the biological yield of the microalgae is increased. Particularly in the cultivation solution in which the macroalgae with the pH value of 6-8 grows properest, according to the method, higher CO2 utilization rate can be guaranteed; meanwhile, the added Tris cannot be consumed by microalgae cells, so that a repeated carbon fixing effect can be achieved, and the economic property is improved. Compared with mono ethanol, Tris nearly does not have a harm effect on the microalgae; when the adding final concentration is 2-8 mmol/L, the biomass yield is increased; furthermore, when the adding final concentration is 4-6 mmol/L, the increase degree is the largest; meanwhile, the biochemical composition of the microalgae cells cannot be affected by the Tris.