Abstract: To investigate the shear characteristics of mixed-fiber-reinforced powdery clay, basalt fiber, polypropylene fiber, and sisal fiber were selected as reinforcement materials. Direct shear tests were conducted on both single-fiber and mixed-fiber reinforced powdery clay. Based on the test data, a weighted combination prediction model was established to explore the intrinsic correlation between single-fiber and mixed-fiber reinforced soils. The results show that: (1) The shear displacement-shear strength relationship curves for both single-fiber and mixed-fiber reinforced soils exhibit strain-hardening behavior. (2) The optimal fiber combinations for mixed-fiber reinforced powdery clay are: 0.1% basalt fiber with 0.3% polypropylene fiber; 0.3% basalt fiber with 0.1% sisal fiber; and 0.2% polypropylene fiber with 0.2% sisal fiber. (3) The shear strength improvement of mixed-fiber reinforced soil lies between that of the corresponding two single-fiber reinforced soils. (4) A weighted combination prediction model for mixed-fiber reinforced soil was established using the weighting method. In this model, the weight assigned to a single fiber type is higher when that fiber exhibits a better reinforcement effect. Comparison between measured shear strengths and model-predicted values confirms the reliability of the weighted combination prediction model. These findings help to establish an intrinsic correlation model between mixed-fiber reinforced soil and the corresponding single-fiber reinforced soils, thereby providing a reference for engineering applications.