Controllable length of pre-crack and in-situ observation are two major difficulties in the research of fracture resistance of ceramic materials. In this study, a method of strain-induced cracking was proposed, in which the plate was bonded to a brass beam to form a composite. The bending deformation of brass beam derived the tension deformation in the tension zone of ceramic thin plate to induce the controllable crack. Four-point bending test was conducted under a tool microscope to perform in-situ observation of crack propagation. The initial crack length was controlled by adjusting the width of the tensile zone of the ceramic plate. After the crack initiation, loading continued to make the crack length meet the requirements of fracture toughness test for ceramics. Compared with standard test of fracture toughness of block materials, the above method is simple and reliable. The strain-induced method to pre-crack has high success rate, the position of crack initiation and the length of pre-crack are controllable and easy to operate. It can be widely applied to fracture toughness evaluation and crack propagation resistance analysis of ultra-thin glass and other ultra-thin brittle materials.