Abstract: The horizontal drag force coefficients are the important mechanical parameters to study the effects of water flow on the scattered underwater riprap stones of the Yellow River dam buttress. And the flow velocities, the root stone size and the elevation angle of the root stones in the water have different effects on the horizontal drag force coefficients. Based on experimental data and FLOW-3D numerical simulation software in this study, RNG k-ε turbulence model, VOF method and FAVOR technique are used to simulate the horizontal drag force coefficients of the underwater riprap blocks under the action of current. Compared with experimental results, the results of numerical simulation are coinciding with the experimental data. The simulated results show that the model is reliable and practical. On the premise of verifying the reliability of the numerical simulation results, the influences of distinct block sizes and different angles between the blocks and current on the horizontal drag force coefficients are analyzed. The numerical simulation results show that when the length-height ratio is not greater than 3, the horizontal drag force coefficients fluctuate and then tend to be stable with the increase of the block length-height ratio. When the length-height ratio is not greater than 3, the horizontal drag force coefficients increase with the increase of the length-height ratio. With the increase of the block width-height ratio, the horizontal drag force coefficients increase first, then decrease and finally remain stable. When the angle between the blocks and the current increases, the horizontal drag force coefficients decrease first and finally increase, and the minimum value appears at the angle of 3 degrees. The results of this numerical simulation can provide a reference for the study of the root stone erosion at the Yellow River dam buttress.