Abstract: Ribulose 1,5-bisphosphate carboxylase /oxygenase(Rubisco,EC 4.1.1.39) is the key photosynthesis enzyme in plants,whose large subunit is encoded by the chloroplast rbcL gene.To better understand the molecular adaption mechanisms of Pteridaceae ferns acclimating to xeric habitats,both adaptive evolutionary and coevolutionary analyses were performed on the rbcL gene sequences of 53 Pteridaceae xeric fern species.The adaptive evolutionary analyses under variable ω ratio sites as well as SLAC,REL and FEL methods indicated that 15 amino acid residues(66S,84E,139L,235G,245I,252A,273Y,295K,296N,299M,307G,330E,349S,365F and 404A) were positively selected.In particular,three sites(245I,252A and 273Y) played an important role in maintaining enzymatic function.The subsequent coevolutionary analysis revealed two groups of coevolutionary residues(139L,273Y and 295K) and(273Y,295K and 349S),which showed that their coevolutionary patterns were significantly related to protein hydrophobicity and molecular weight.This research consolidated that evolutionary models using ω as an indicator of selective pressure provide valuable analysis of the adaption of protein encoding sequences.In addition,this study also demonstrated that Pteridaceae ferns may respond to xeric habitats by adaptively modifying the rbcL gene.