To study the response of mode I cracks under impact loading, the dynamic fracture test on the single cleavage triangle (SCT) granite specimens with the lateral opening was carried out using a split Hopkinson pressure bar test system. The Hough transform method was used to quantitatively describe the distributions of the surface crack lengths and angles, and the relationship between them and the absorbed energy of the specimen was analyzed. The three-dimensional (3D) point cloud data of the fracture surface was obtained by using a 3D surface topography detection device, and a fracture surface reconstruction method based on the threshold detection of the fitting surface was proposed, which effectively solved the error problem caused by the appearance of error points in the 3D point cloud data of the fracture surface. The relationship between fracture surface roughness and absorbed energy was also discussed. The results show that the exponential distribution of crack length λ tends to increase, and the distribution of crack angles is more even with the increase in absorbed energy. There are relatively few cracks in the horizontal direction, and the cracks have obvious directionality when the dissipated energy of the specimen is lower, while the bending degree of the cracks is higher and the connectivity is better when the dissipated energy is higher. The fracture surface reconstruction method can complete the fracture surface processing better at 0.03-0.08 of the points in the x and y directions and the threshold of 0.25. The roughness statistical parameters of fracture surfaces A and B showed a decreasing trend with the increase in dissipated energy.