Ferns that evolved from 400 million years ago show various functional traits and ecological strategies in extant species, over 80% of which belong to the youngest order Polypodiales. How the functional traits and strategies of ferns have changed during their evolutionary history remains unexplored. Here, we measured functional traits that are sensitive to environmental light and water availability in 345 fern species across fern phylogeny, and reconstructed their evolutionary histories. We found that ferns, mainly Polypodiales, have developed diversified functional traits in response to forest environments. Terrestrial species, especially Thelypteridaceae and Athyriaceae in eupolypods II, since the late Jurassic period, have shown decreased leaf mass per area (LMA) and area‐based leaf nitrogen (Narea), but increased mass‐based leaf nitrogen (Nmass) compared with early‐derived polypods. Epiphytic species, mainly those in Polypodiaceae, have shown reductions in Nmass and individual leaf area (Area) since the late Cretaceous period. The adaption of functional traits of Polypodiales to forest environment may have played a crucial role in fern radiation since the late Jurassic period. Integrative analysis of functional traits, especially numerical ones, may shed new light on plant evolution. Phylogenetic analysis of functional traits from extant ferns revealed how ferns have adapted to forest environments, including the shade and moist forest floor and the exposed and xeric forest canopy.