Preform design plays an important role in forging design especially for parts with complex shapes. In this paper, an attempt was made to develop a topological optimization approach for the preform design in bulk metal forming processes based on the bidirectional evolutionary structural optimization strategy. In this approach, a new strain-based element addition and removal criterion has been proposed for evaluating and optimizing the material flow in the forging process. To obtain a smooth preform boundary, a closed B-spline curve based on the least square algorithm is employed to approximate the uneven surface of the updated preform profile. A C# program has been developed to integrate the FE simulation, shape optimization, and surface approximation processes. Two 2D forging preform design problems are evaluated by using the developed method. The results suggest that the optimized preform with the strain uniformity criterion has shown better performance in improving the material flow and deformation uniformity during the forging process. The results also demonstrate the robustness and efficiency of the developed preform optimization method.