The invention provides a method for strengthening a light fiber composite material. The method includes the steps that plant fiber is taken to be crushed and pretreated to obtain a plant fiber raw material; the plant fiber raw material is assembled and hardened by heating to prepare a semifinished product, wherein in the assembling process, blended fiber obtained after gluing is quantitatively and uniformly laid in a forming frame, or the plant fiber is mixed with the blended fiber to be formed through air spreading needling; the semifinished product is soaked with a foaming preparation and then subjected to high-temperature setting and surface reprocessing, and a flat plate or the heterotypic composite material is prepared, wherein the foaming preparation is quantitatively prepared according to production design requirements, in the soaking process, the foaming preparation penetrates into the semifinished product, the foaming preparation at least includes foamable phenolic resin, a foaming agent and a surfactant, and one or more of a curing agent, a modifier and a flame retardant can be added in the foaming preparation. The method can be used for making a high-strength artificial light composite board, the prepared fiber composite board is low in density and good in mechanical property.