investigating the mechanism of 3d crack propagation and coalescence in rock under compression by mechanics experiment is an important means to understand the damage mechanism of instability of rock mass. Because of the real rock with internal cracks cannot be directly observed, the uniaxial compression tests on transparent rock-like material which is developed by myself are done on the RMT-150B multi-function automatic rigid rock servo material testing machine to observe and study the mechanism of 3d cracks propagation and coalescence process which are in the transparent rock-like material. The method overcome the character of real rock is not transparent and is convenient to observe the internal crack initiation of rock mass. Through setting the different Angles and lengths of pre-existing crack to simulate influence on the instability of rock mass damage in different Angle and length of the original joints. The results showed that 3 d crack propagation and coalescence process is much more complex than that of 2d crack, the failure process of the simple can be divided into four stages which are compression, elastic deformation, crack propagation and brittle failure. Compared with the intact sample, the peak strength and peak axial strain of the sample with pre-existing are all lower, and the reduction extent is closely related to the length of the pre-existing crack. The research result offers great reference value to analysis the mechanism of rock damage and instability.