To investigate the dynamic mechanical properties and failure mode size effects of fractured rocks, different sizes of pre fabricated fractured rock samples were taken as the research objects. Through SHPB tests with the same impact strength, the correlation and degree of correlation between sample size effects and strain rate effects were obtained. Based on multiple sets of confirmatory SHPB tests, the mechanical properties and failure mode characteristics of different size samples under the same strain rate conditions were obtained. The research results indicate that the size effect and strain rate effect have a coupled effect on the dynamic strength of the specimen. Under the same strain rate conditions, the dynamic strength of the specimen increases with the increase of size. Compared with the results of the same impact strength test, controlling the same strain rate will significantly weaken the influence of the pre fabricated crack inclination angle on the dynamic strength. Under the same strain rate, the first newly formed crack in the specimen generally initiates at the end of the pre-existing crack. In this study, under the strain rate conditions, as the specimen size increases, the types of cracks in the specimen change from tensile, shear, and mixed cracks to mainly tensile cracks. The fragmentation degree of the sample is related to the type of crack propagation. The average fragmentation degree of the sample is a sensitive variable that varies with the size of the sample. As the size of the sample increases, the fractal dimension of the sample fragments gradually increases. Different pre fabricated crack angles have different effects on the fractal dimension, which is closely related to the type and quantity of cracks in the sample.