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To investigate the dynamic mechanical properties of water-saturated fissure rock at different strain rates, prefabricated sandstone specimens with a 45° dip angle were treated with water saturation and the impact compression test was performed with a Split Hopkinson Pressure Bar (SHPB) test device at different impact pressures. The results show that the clusters of dynamic stress–strain curves of water-saturated and natural sandstone specimens with a 45° dip angle of prefabricated fissures are basically similar under different impact air pressures. A distinct strain rate effect was observed for dynamic strain and dynamic compressive strength, both of which increased with increasing strain rate. From the failure pattern of the specimen, it can be seen that cracks appeared from the tip of the prefabricated fissure under axial stress, spreading to both ends and forming wing cracks and anti-wing cracks associated with shear cracks. As the strain rate increased, the energy dissipation density of the specimen gradually increased, and the macroscopic cracks cross-expanded with each other. The fracture form of the specimen showed a small block distribution, and the average particle size of the specimen gradually decreased. The specimen crushing energy dissipation density was negatively correlated with fracture size, reflecting a certain rate correlation. The sandstone fragments’ fractal dimension increases with the increase in crushing energy dissipation density, and the fractal dimension may be applied as a quantitative index to characterize sandstone crushing.