The invention provides a method for reinforcing and gripping at an end part of a test piece during stress relaxation test of an FRP rib. A gradient transition section is arranged at a reinforcing section at the end part of the test piece, and an external thread arranged at the reinforcing section is matched with a screw thread of a metal splint with an internal thread to eliminate sliding and creeping of a gripping area at the end part during experiment. The method is specifically realized by the following steps: intertwining a section of fiber cloth soaked by resin and being capable of realizing gradient transition at the end part of the test piece, winding the external thread at the glass fabric intertwined section by a thin fiber strand according to a predetermined thread pitch before resin solidification, matching the external thread with the internal thread of the metal splint, and buckling two prefabricated metal splints coated with release agent to the wound screw thread to be fixed. The method solves the problem that sliding, creeping and the like in the gripping area at the end part of the test piece influence the test results when the FRP rib is subjected to stress relaxation test, and the metal splints can be repeatedly utilized.