Determination of the drift distance of a riprap is critical for water riprap works. Most of researchers have focused on the drift distance of a single particle rock so far, and a systematic study on the drift distance of a rock group was rarely carried out. It remains to be verified whether the drift distance of a rock with an average mass can represent that of a rock group. In this paper, a series of experiments have been conducted to investigate the drift and diffusion of a rock group with different mass combinations under different water depths and flow velocities. Comparisons between the calculated drift distance of the rock with an average mass using the formula for a single particle rock and the average drift distance of the rock group from the experiments were curried out. The results show that the drift distanceof the rock with an average mass is close with that of the rock group if the water depth is less than 15m while the drift distance of the rock with an average mass is greater than that of the rock group if the water depth is around 30m. Furthermore, a formula for calculating the distribution area of a rock group riprap was derived through dimensional analysis. Reasonable agreement of the calculated results between the model experiment and field observations verified the reliability of the calculating formula.