Abstract: The optimum anchorage angle is the important parameter in the design of anchoring engineering. In order to study the effect of bolt length and slope rate on the optimum anchorage angle, a numerical model for slopes is established using FLAC^3D. It is found that the anchor length and the slope rate have great influence on the optimum anchorage angle. The results show that: (1) With the increase of anchor length, the factor of safety decreases at first and then remains stable, so there exists an effective length. The effective length decreases with the increase of anchorage angle. (2) The increase of anchorage angle will lead to the safety factor first to increase and then decrease, so there exists an optimum anchorage angle. The optimum anchorage angle will decrease linearly with the increase of bolt length. (3) When the anchorage angle is less than the optimum anchorage angle, the slope rate only has small impact on the safety factor. When the anchorage angle is the same as the optimum anchorage angle or larger than the optimum anchorage angle, the impact of the slope rate on the slope stability becomes more obviously, and the larger slope rate leads to the larger optimum anchorage angle.