Abstract: Fast and accurate quantitative detection of ¹⁴CO₂ has important applications in many fields. The optical detection method based on the sensitive cavity ring-down spectroscopy technology has great potential. But currently it has difficulties of insufficient sensitivity and susceptibility to absorption of other isotopes/impurity molecules. We propose a stepped double-resonance spectroscopy method to excite ¹⁴CO₂ molecules to an intermediate vibrationally excited state, and use cavity ring-down spectroscopy to probe them. The two-photon process significantly improves the selectivity of detection. We derive the quantitative measurement capability of double-resonance absorption spectroscopy. The simulation results show that the double-resonance spectroscopy measurement is Doppler-free, thereby reducing the effect of other molecular absorption. It is expected that this method can achieve high-selectivity detection of ¹⁴CO₂ at the sub-ppt level.