Abstract: Shear failure of weak interface as natural cracks or bedding is the key factor for the formation of crack network. Based on the principle of a three-point bending test, this paper sets up the test bed of mode I crack extension weak plane. The digital image correlation method is used to obtain the specimen surface displacement and strain field changes when the crack extends through the weak plane. The experiment results show that: when the crack extended at the weak interface, the extension stopped transitory, the COD (crack opening displacement) increased rapidly, the crack blunted, the shear strain increased rapidly, the mode I crack transforms to a mixed mode I-II crack and the crack extended deflective.