Abstract: This paper introduced COD gage instead of traditional strain gage to control the strain in front of the crack for cantilever bend loading style. Through the FEM analysis, the influence of COD gage's fixing position and form to the symmetry of cantilever bend loading specimen (GROSS specimen) was researched. The results indicated that the greater the COD gage's edge opening was, the smaller the monitor points' unsymmetrical strain aberration showed; if the edge's width was fixed, the perturbation strength of the monitor points' symmetrical strain would be reduced along with the decrease of the edge peak's angle. Considering the measuring range and stability of the COD gage, the actual specimen was designed as 10 mm for the width of the COD gage's fixing edge, 15° for the root segment. The linearity relationship was obeyed for the fixing edge's relative displacement and monitor points' strain by using COD gage to achieve strain controlling test. Therefore, it was considered that this improvement for strain controlling method of cantilever bend loading was reliable,reasonable,stable for controlling and convenient for operation.