Abstract: Water has a great influence on the physical and mechanical properties of the rock. It is very important to investigate the fractal law of rock crushed under impact load at different moisture contents. Using the Split Hopkinson Pressure Bar(SHPB),the impact tests of argillaceous siltstone with different water contents are carried out. The fragmentation distribution of the broken rock is thoroughly presented. According to the method of Size-Frequency, the fractal law of particle size distribution is analyzed. It shows that, the higher the impact velocity, the larger the volume of the singe fragments, the more the amount of the fragments of argillaceous siltstone, the smaller the fractal dimension. The greater the moisture content, the greater the proportion of small particles, the higher the degree of crushing. The fractal dimension of water-saturated rock is bigger than the samples in other two states. The fractal dimensions both in the natural moisture state and natural water absorbing state are almost the same. When the impact velocity is higher, it can become the main factor that affects the fractal dimension, and the influence of the moisture content is smaller.