Abstract: The most obvious difference between Spirulina medium and other algal medium is the high concentration of HCO₃ and CO₃^2- in Spirulina medium. The influence of carbon concentration.temperature,pH-value,salinity and movement of medium upon the CO₂ absorption rate are investigated quantitatively,and the mechanism are explained by using the physical model of absorption of CO₂ by Spirulina medium. Whether the chemical reaction influences the CO₂ absorption rate has been a contentious question, this question is discussed in this paper. It is pointed out that the chemical reaction certainly influences the CO₂ absorption rate,but whether the influence is observed or not mainly depends on the movement of medium.The theoretic value of working area is calculated according to the desired productivity,carbon concentration and movement of medium in the case of supply CO₂ by means of"gasometer". By using the concept of medium's "CO₂ capacity", it is demonstrated that higher carbon conversion ratio can be achieved,and more benefite can be got to produce Spirulina by using CO₂ as carbon source compared with other kind of algae.