The invention relates to a composition based on an impact-resistant polyamide thermoplastic resin, comprising, as a blend: from 60 to 98% by weight of polyamide resin (A) and from 2 to 40% by weight of a blend of at least one polyamide-block graft copolymer (B) composed of a polyolefin backbone and of at least one polyamide graft in which the grafts are attached to the backbone by the residues of an unsaturated monomer (X) having a functional group capable of reacting with an amine-terminated polyamide, the residues of the unsaturated monomer (X) being attached to the backbone by grafting or copolymerization via its double bond, and of a non-reactive ethylenic polymer (C) that is miscible with (B) and has a density of less than 0.960 g/cm3. This composition has improved impact resistance properties and also satisfactory melt flow.The invention also relates to the method of obtaining said composition and also to its use for manufacturing products or parts obtained by injection moulding or extrusion.