In order to meet the needs of bridge structure health monitoring, the home-made long-gauge fiber Bragg grating (FBG) macro strain sensor is used to measure average shear strain and identify occurrence of oblique cracks. The FBG is used as a measuring element and five couples of crossed topologies are designed. We use the cross topological theory to measure different parts of concrete T-beam, and analyze relationship between the average shear strain of the structure and load in crossed topological coverage. The experimental results show that the measuring method of the long-gauge FBG crossed topology is effective to identify occurrence and development of oblique cracks, and this method has good application value in engineering practice.