Interlaminar shear strength is one of key mechanical properties for fiber reinforced composite laminates, which is mostly evaluated by the three-point bending method using short beam. However, the short beam specimen probably failing in multiple modes may lead to an unsuccessful testing. In this paper, a long-beam specimen scheme is proposed to improve three-point bending testing method. The span to height ratio of the proposed long beam specimen is adjusted by sticking reinforced patches to the surfaces of laminated specimen, which in turn lowers the ratio of the maximum normal stress and the maximum shear stress within the long beam during three-point bending. By this way, the elevated interlaminar shear stress will lead to shear failure of the specimen to guarantee the effectiveness of testing. Based on the deduced stress formulas of the proposed long beam, the feasibility of long beam with reinforced patches to evoke interlaminar shear failure is discussed. The shear stress formulas are then verified by numerical analysis. Finally, the interlaminar shear strength of glass-fiber reinforced laminates is measured by the proposed testing method, which shows that all the specimens fail in the interlaminar failure mode. Therefore, the proposed testing method of long beam is reliable for evaluation of interlaminar shear strength.