The invention relates to a testing device for hull shells, wherein the test specimen is joined to a complementary structure with legs, having a U-shape in cross section, and at the ends of the legs there are provided lengthwise flanges in the lengthwise direction of the complementary structure with means of attaching the hull shell to the complementary structure and of applying and absorbing corrective forces, while at the borders of the complementary structure in the lengthwise direction there are provided transverse flanges for applying and absorbing of test forces in six degrees of freedom. The complementary structure (1) is formed as a soft box of composite materials and the U-shaped cross section of the complementary structure (1) consists of a horizontal straight segment (11) and two ellipsoidal segments (12) adjoining each side, forming the legs (3) of the U-shaped complementary structure (1).