Abstract: Three point bending beam fracture experiments were carried out by means of transmission type static caustics method to study the interaction of difformity inclusions and matrix crack. Firstly, the caustic patterns of mode-I crack tip of difformity inclusions were obtained, and then the β value was introduced, which was defined as the ratio of vertical and horizontal axis of caustic pattern, reflecting the distortion performance of caustic pattern under the effect of inclusion. Secondly, the corresponding feature dimensions of caustic pattern were extracted, and the stress intensity factor K_I of mode-I crack tips were obtained. Finally, the impact of difformity inclusions on stress field singularity around the crack tip was revealed based on the relationship of caustic pattern, stress intensity factor and the distance between crack tip and inclusion. The experiment provides reference for strength design and fracture performance assessment of structures containing inclusion.