This paper summarizes the latest approaches and results of reconstructed paleo-CO₂ level for the pre-Quaternary period. The theory and process of the new proxy are clarified and its corresponding mathematical model, BRYOCARB, is introduced. Furthermore, in order to test the result of the new method, a case study has been carried out. The results show a agreement with other proxies and the long term geochemical models, which proves the carbon isotope discrimination of fossil liverwort, is an effective proxy.