To investigate mesoscopic cracking propagation and coalescence mechanisms of rocks under shearing conditions, tests on sandstone samples are conducted using a self-developed mesoscopic test equipment. The effects of saturation degree on mesoscopic cracking and crack morphology are discussed. It is shown that in general cracks initially appear at the middle of the shear plane with stress dropping during the shearing process. Several cracks distribute discontinuously, and cracks within a short distance coalesce into large cracks. At the moment of stress dropping, all cracks connect with each other into a coalesced surface. The rock strength is affected by water weakening, which leads to large secondary cracks appearing at the bottom of specimens. When the saturation degree is higher, there are more bifurcation cracks appear around the main crack, and the specimen has more complicated surface patterns. When the number of fractured surface is increased, the secondary crack is growing slower and overall damage area is increasing faster. From the mesoscope aspects, the local independent crack coalescence process is accompanied by the beam rotation and bending fracture of the cemented rock bridge; the weakening effect of water results in the internal and external microcracks in the rock bridge increase. With the increase of saturation degree, the numbers of both independent cracks and secondary cracks show an increasing trend during shear process.