The invention discloses salvia splendens petal-based porous graphene-like carbon nanosheets as well as a preparation method and an application thereof. The porous carbon nanosheets with graphene-likestructure are prepared from salvia splendens petals as a biomass precursor based on an encapsulation protection strategy. Firstly, the salvia splendens petals and a saturated encapsulant solution areuniformly mixed, then the mixture is pyrolyzed at high temperature under the protection of inert gas, and is also encapsulated and protected during pyrolysis, and the porous graphene-like carbon nanosheets are formed. The prepared porous carbon nanosheets have a graphene-like two-dimensional sheet structure, large specific surface area and strong conductivity; when used as an electrode material for a supercapacitor, the porous carbon nanosheets have high capacitance performance, good rate capability and long cycle life. The method is simple to operate, safe, environmentally friendly, economic,efficient and capable of realizing large-scale production of the porous carbon nanosheets.