Objective To compare differences in mechanical stability of intramedullary fibular allograft with cannulated screw (modified method) and cannulated screw alone (conventional method) for fixing young and middle-aged Pauwels Ⅰ, Ⅱ, Ⅲ femoral neck fractures. Methods Models of Pauwels Ⅰ, Ⅱ, Ⅲ femoral neck fracture fixed by conventional method and modified method were constructed. Stress distributions on weight-bearing area of the femoral cortical bone shell and the end of femoral neck fracture, as well as shear stress distributions on cortical bone shell of the femoral head and femoral neck fracture surface were analyzed, the maximum principal strain cloud maps of the femur in coronal position were drawn according to the predicted data, and the displacements of femoral neck fracture end between two groups were compared. Results The shear stress distributions on cortical bone shell of the femoral head in two directions (S12, S13) showed that femoral neck fractures fixed by modified method was superior or close to that by conventional method. Besides, the shear stress distributions on fracture surface of the femoral neck in two directions (S12, S23) showed that modified internal fixation was superior to conventional internal fixation. The displacements of femoral neck fracture end in Pauwels Ⅱ and Ⅲ fracture fixed by conventional method were greater than those by modified method and the displacements of Pauwels Ⅱ fracture fixed by conventional method were obviously larger than those of Pauwels Ⅱ fracture fixed by modified method. But Pauwels Ⅰ fracture fixed by modified method showed a larger displacement than that fixed by conventional method. Conclusions Modified method is more suitable for fixing femoral neck fracture with large angles (Pauwels Ⅱ and Ⅲ fracture), and conventional method is more suitable for fixing neck fracture with small angles (Pauwels Ⅰ fracture).