The pumpability of coal gangue slurry is affected by its rheological properties, which are regulated by particle size and water/gangue ratio. The rheological properties, degree of fluidity and flow time of fresh coal gangue slurry were tested in this paper, and the effects of different particle sizes (100 mesh, 150 mesh, 200 mesh, 250 mesh, 300 mesh, corresponding to 150 μm, 106 μm, 74 μm, 58 μm and 48 μm, respectively) and water/gangue ratio (1.0, 1.5, 2.0) on the rheological parameters were discussed. The variation rules of rheological parameters, degree of fluidity and flow time were discussed as well, and some grouting suggestions were put forward. The results show that the water/gangue ratio is the main controlling factor of rheological properties of coal gangue slurry. When the water/gangue ratio is 1.0, the coal gangue slurry conforms to the Herschel-Bulkley model, and the particle size affects both the yield stress and plastic viscosity of coal gangue slurry, and the yield stress is between 2.5 Pa and 3.6 Pa. When the water/gangue ratio is 1.5 and 2.0, the coal gangue slurry conforms to the Bingham model, and the particle size only affects the yield stress of coal gangue slurry, and the yield stress is between 0.1 Pa and 0.7 Pa. In addition, the degree of fluidity of coal gangue slurry with different particle sizes is more than 365 mm, and the flow time is in the range of 27 s to 31 s. Based on these test results, grouting suggestions are put forward that a larger water/gangue ratio (≥1.5) and a longer stirring time (>800 s) are recommended to improve the pumpability of coal gangue slurry.