In this paper, the molar absorptivity of carbonate ions is calculated by measuring the infrared absorption spectra of 30-100 mmol/L low-concentration sodium carbonate aqueous solutions. The infrared absorption spectra of 30-100 mmol/L sodium carbonate aqueous solutions were measured using the infrared single attenuated total reflection (ATR) accessory based on zinc selenide. The simulated pure water ATR spectrum was used as the baseline to adjust the infrared absorption spectra of the sodium carbonate aqueous solutions. The Kramers-Kronig formula was used to calculate the refractive indices and extinction coefficients of the sodium carbonate solutions. Furthermore, the effective optical pathlengths and the molar absorptivities of the low-concentration sodium carbonate aqueous solutions were calculated. The calculated maximum molar absorptivity of carbonate ions is 2421.99 L/(mol·cm).