In the CO2 transcritical power cycle, conventional cooling water can hardly condense subcritical CO2 because its critical temperature is as low as 30.98 degrees C. In order to avoid this condensing problem, CO2-based mixtures have been proposed as working fluids for transcritical power cycle. They can raise the critical temperature by mixing a little C3H8 as the secondary component to CO2. However, the flammability of the mixture may limit its application. This article investigated laminar flame speed of C3H8/CO2 which represents the mixture's combustion characteristic by a so-called heat flux method and studied the inhibition mechanism of CO2 on the combustion based on the Premixed Laminar Flame-Speed Calculation reactor of Chemkin-Pro. The experimental results showed that the laminar flame speed shows a peak value with changing the equivalence ratio and accelerates with raising the mole fraction of the organic gas. Additionally, a slight upwards trend was observed for the corresponding equivalence ratio of the peaks. The flammable range for the equivalence ratio extended with the mole fraction of C3H8 increasing. With the mole fraction of C3H8 of 0.15, the maximum laminar flame speed was 12.8 cm/s, 31.7% of that of the pure C3H8. The flammable range was from 0.41 to 1.33, decreasing by 23.3% compared with that of C3H8. A flammable critical mixing ratio was also found as 0.08/0.92 for C3H8/CO2 at the normal condition. By simulating, it was found that the most key free radical and elementary reaction which determine the inhibition of CO2 on the combustion are OH and H+O-2=O+OH, respectively.