Borehole pressure relief is a widely adopted prevention and control measure to mitigate the risks of high-stress dynamic loads on roadways. An appropriate pressure relief hole diameter is of great significance for the protection of roadways against rock bursts. To investigate the influence of pressure relief hole diameter on the dynamic response characteristics of surrounding rock under dynamic loads, a split Hopkinson pressure bar (SHPB) test system was used to conduct dynamic impact tests on prefabricated borehole specimens containing different hole diameters. The effects of pressure relief hole diameter on the dynamic mechanical properties, failure characteristics, and energy dissipation of the specimens were analyzed. The test results reveal that an increase in hole diameter leads to a reduction in peak stress and peak strain. Specifically, the reduction amplitude of peak stress gradually decreases as the hole diameter increases, while the reduction amplitude of peak strain gradually increases. Under dynamic load, the final failure mode of all specimens containing holes is dominated by X-shaped tensile-shear composite failure. However, the angle between the tensile-shear composite cracks and the loading direction, the development and expansion degree of initial tensile cracks before closure, and the development degree of far-field cracks are affected by the hole diameter. The presence of holes indirectly provides stress conditions for the development of fractures in the specimens. Therefore, the fractal dimension increases with the increase of hole diameter. However, when the hole diameter exceeds a certain threshold value, the specimens have been destroyed before the development of fractures, leading to a decrease in the fractal dimension. The energy dissipation ratio of the specimens is similar to their fragmentation characteristics, showing a trend of first increasing and then decreasing with the increase of hole diameter. This phenomenon is mainly related to the delay effect caused by different hole diameters. The research results provide a certain guiding significance for the optimization of pressure relief hole diameter in deep roadways threatened by dynamic loads.