Experimental studies are preformed to investigate the mechanism of 3D crack propagation and coalescence in rock under compression, which is significant to understand the damage mechanism and instability of rock mass. Since internal cracks in rock cannot be directly observed and the CT scanning technique is lack of real-time capability, a type of transparent rock-like material is developed to investigate the mechanisms of 3D cracks propagation and coalescence. This transparent material has similar cracking behaviour as rock material, and thus it is convenient to observe the internal crack initiation and propagation. Uniaxial compression tests are conducted on transparent specimens containing build-in single and double pre-exciting cracks using a RMT-150B multifunction automatically rigid servo testing machine. The patterns of 3D cracks propagation and coalescence of specimens with different bridge angles and crack spacings are obtained by detailed observation. Additionally, the influence of different preexisting crack number and crack spacings on peak strength is examined. At last, a theoretical explanation is presented to explain the crack propagation process. The results show that the propagation and coalescence modes of secondary cracks are controlled by different bridge angles and crack spacings. The secondary cracks included wing crack, anti-wing crack (growing in opposite direction with wing crack), and petal-shaped crack (caused by tensile and shear stress). The coalescence of all kinds of cracks leads to the final failure of specimens. The existence of cracks significantly reduces the compressive strength of specimens, and as the number of crack increases, the peak strength exhibits a trend of decline. Furthermore, the crack spacing also affects the peak strength. This research results provide a valuable guide for analyzing the mechanism of rock damage and instability.