A method of producing a composite material, comprising an assembly of natural fibers, impregnated with an amorphous or semi-crystalline thermoplastic polymer, said method including: i) a step of impregnating said assembly with a precursor composition in the molten state and including: a) at least one prepolymer P(X)n of said thermoplastic polymer, having a molecular chain P having, at the ends of same, n reactive functions X, with n ranging from 1 to 3, in particular n being 1 or 2, and preferably 2 b) at least one chain extender Y-A-Y including reactive functions Y with at least one of said functions X ii) a step of polymerization by mass (poly)addition, in the molten state, of said prepolymer with said chain extender with: said thermoplastic polymer being the result of said polymerization by mass polyaddition and said impregnation i) and polymerization ii) taking place at a temperature lower than 250° C.