Abstract: The mode I double cantilever beam (DCB) test was commonly applied to investigate the material resistance to crack propagation in unidirectional composites, aiming at obtaining the interlaminar fracture toughness in mode I, which was an important input parameter for the study of delamination propagation and failure mechanism of composite materials. The DCB test must be suspended frequently for the multiple measurements of the crack length, which will not only have a potential effect on the propagation of crack and even lead to the measurement error, but also can be a time and effort consuming process. Digital image correlation (DIC) technology applied to crack propagation length measurement has the advantages of real-time tracking and precise positioning, effectively improving the measurement efficiency of the mode I fracture toughness, but it still has limitation when applied to discontinuous deformation behavior, and it is susceptible to interference from image noise, resulting in measurement error. This paper developed a real-time crack length detection method based on DIC and obtained the discontinuous deformation displacement field of the specimen through an image matching algorithm, and then proposed an identification method based on the degree of dispersion of the global lateral displacement, which realized the crack tip real-time capture. Then, compared with the traditional measurement method in the DCB test, the measurement error of the crack length does not exceed 2.76% on average, which verifies the accuracy and efficiency of the method, meanwhile, overcomes the measurement interference caused by roughness of the side surface, the poor speckle quality and the fiber bridging of the poly p-phenylene-2,6-benzoxazole (PBO)/epoxy composites. Finally, the effective initial value and steady-state propagation value of the mode I interlaminar fracture toughness.