The objective of this study was to estimate the $CO_2$ absorptivity provided by an in situ carbon supply system using a photosynthetic culture of the cyanobacterium Spirulina platensis in an open raceway pond. The effects of initial total carbon concentrations (ranging from 0 to 0.1 mol/L), suspension depths (ranging from 5 to 20 cm) and pH values (ranging from 8.9 to 11.0) on the $CO_2$ absorptivity were studied. The results indicated that $CO_2$ absorptivity was positively correlated with pH value, negatively correlated with total carbon concentration, and only negligibly affected by the suspension depth. The optimum total carbon concentration range and pH range were 0.03 ~ 0.09 mol/L and 9.7 ~ 10.0, respectively. An average $CO_2$ absorptivity of 86.16% and average $CO_2$ utilization efficiency of 79.18% were achieved using this in situ carbon-supply system in large-scale cultivation of Spirulina platensis, with an initial total carbon concentration of 0.06 mol/L and pH value of 9.8. Our results demonstrated that this system could obtain a favorable $CO_2$ utilization efficiency in outdoor, large-scale cultivation of Spirulina platensis in open raceway ponds.