The slake durability; grain quantity, size, roundness, and slake degree of red-bed mudstones are experimentally investigated using the slake durability test and the cyclic wetting and drying test. Results indicate that the complete slaking process exhibits three distinct stages. During the process, the slake durability index decreases linearly with increasing cycle number. Slight variation in grain quantity is observed. Smaller grain size fractions tend to have higher slaking degree indexes in earlier cycles. Grain roundness fluctuates. Two parameters, namely, fractal dimension and surface energy, are revealed to have the ability to represent the degree of slaking of red-bed mudstones.