To realize the deformation field measurement of mixed mode crack tip and failure mode analysis of hydroxyl-terminated polybutadiene (HTPB) propellant, HTPB propellant specimens with central through-mixed mode cracks were made. Tensile observation test was carried out. The propagation characteristics of the mixed mode crack were obtained. Digital image correlation(DIC) method was used to obtain the strain field on the surface of the specimen and the crack tip. The characteristics of the strain field near the mixed mode crack tip and the relationship between the strain field and the crack propagation law were studied. The results show that the tensile process of mixed mode crack specimens can be divided into three stages: linear segment, nonlinear segment and failure segment, and the crack propagates in the direction perpendicular to the load. The digital image correlation method using large deformation analysis method can effectively solve the problem of large deformation of the specimen, and can quantitatively give the strain field on the surface of the specimen and the strain concentration area is consistent with the theoretical results. The expansion of mixed mode cracks is closely related to the change of the strain field. The strain field generates strain concentration at the crack tip, leading in crack propagation.